Seto Inland Sea in the context of Sea of Japan

The Taiheiyō Belt (Japanese: 太平洋ベルト, Hepburn: Taiheiyō Beruto; lit. "Pacific Belt"), also known as the Tōkaidō corridor, is the megalopolis in Japan extending from Ibaraki Prefecture in the northeast to Fukuoka Prefecture in the southwest, running for almost 1,200 km (750 mi). Its estimated population as of 2011 was about 80 million.

The urbanization zone runs mainly along the Pacific coast (hence the name) of Japan from Kantō region to Osaka, and the Inland Sea (on both sides) to Fukuoka, and is concentrated along the Tōkaidō–Sanyō rail corridor. A view of Japan at night clearly shows a rather dense and continuous strip of light (demarcating urban zones) that delineates the region. The high population is particularly due to the large plains – the Kantō Plain, Kinai Plain, and Nōbi Plain – which facilitate building in otherwise mountainous Japan.

Japan has the eighth-largest exclusive economic zone (EEZ) in the world. The total area of Japan is about 380 thousand km. Japan's EEZ area is vast and the territorial waters (including the Seto Inland Sea) and EEZ together is about 4.47 million km.

Hyōgo Prefecture (兵庫県, Hyōgo-ken; Japanese pronunciation: [çoꜜː.ɡo, -ŋo, çoː.ɡoꜜ.keɴ, -ŋoꜜ.keɴ]) is a prefecture of Japan located in the Kansai region of Honshu. Hyōgo Prefecture has a population of 5,469,762 (as of 1 June 2019) and a geographic area of 8,400 square kilometres (3,200 square miles). Hyōgo Prefecture borders Kyoto Prefecture to the east, Osaka Prefecture to the southeast, and Okayama and Tottori prefectures to the west.

Kobe is the capital and largest city of Hyōgo Prefecture, and the seventh-largest city in Japan, with other major cities including Himeji, Nishinomiya, and Amagasaki. Hyōgo Prefecture's mainland stretches from the Sea of Japan to the Seto Inland Sea, where Awaji Island and a small archipelago of islands belonging to the prefecture are located. Hyōgo Prefecture is a major economic center, transportation hub, and tourist destination in western Japan, with 20% of the prefecture's land area designated as Natural Parks. Hyōgo Prefecture forms part of the Kobe metropolitan area and Osaka metropolitan area, the second-most-populated urban region in Japan after the Greater Tokyo area and one of the world's most productive regions by GDP.

Wakayama Prefecture (和歌山県, Wakayama-ken; Japanese pronunciation: [wa.kaꜜ.ja.ma, wa.ka.ja.maꜜ.keɴ]) is a prefecture of Japan located in the Kansai region of Honshu. Wakayama Prefecture has a population of 876,030 (as of 1 February 2025) and a geographic area of 4,724 square kilometres (1,824 sq mi). Wakayama Prefecture borders Osaka Prefecture to the north, and Mie Prefecture and Nara Prefecture to the northeast.

Wakayama is the capital and largest city of Wakayama Prefecture, with other major cities including Tanabe, Hashimoto, and Kinokawa. Wakayama Prefecture is located on the southwestern coast of the Kii Peninsula on the Kii Channel, connecting the Pacific Ocean and Seto Inland Sea, across from Tokushima Prefecture on the island of Shikoku.

Osaka Bay (大阪湾 Ōsaka-wan Japanese: [oːsakaꜜwaɴ]) is an inland bay in western Japan, on the eastern side of the Seto Inland Sea. It is separated from the Pacific Ocean by the Kii Channel and from the neighbor western part of the Inland Sea by the Akashi Strait. Its western shore is formed by Awaji Island, and its northern and eastern shores are part of the Kansai metropolitan area.

Major ports on Osaka Bay include Osaka, Kobe, Nishinomiya, Sakai, Amagasaki, and Hannan.

Fukuoka Prefecture (福岡県, Fukuoka-ken; Japanese pronunciation: [ɸɯ̥.kɯꜜ.o.ka, -kɯ.o.kaꜜ.keɴ]) is a prefecture of Japan located on the island of Kyūshū. Fukuoka Prefecture has a population of 5,109,323 (1 June 2019) and has a geographic area of 4,986 km (1,925 sq mi). Fukuoka Prefecture borders Saga Prefecture to the southwest, Kumamoto Prefecture to the south, and Ōita Prefecture to the southeast.

Fukuoka is the capital and largest city of Fukuoka Prefecture, and the largest city on Kyūshū, with other major cities including Kitakyushu, Kurume, and Ōmuta. Fukuoka Prefecture is located at the northernmost point of Kyūshū on the Kanmon Straits, connecting the Tsushima Strait and the Seto Inland Sea across from Yamaguchi Prefecture on the island of Honshu, and extends south towards the Ariake Sea.

The San'yō Main Line (山陽本線, San'yō-honsen) is a major railway line owned by JR Group companies in western Japan, connecting Kōbe Station and Moji Station, largely paralleling the coast of the Seto Inland Sea, in other words, the southern coast of western Honshu. The San'yō Shinkansen line largely parallels its route. The name Sanyō derived from the ancient region and highway San'yōdō, the road on the sunny (south) side of the mountains.

The Sanyō Main Line is operated by two JR companies:

Hiroshima Prefecture (広島県, Hiroshima-ken; Japanese pronunciation: [çi.ɾo.ɕi.ma, -maꜜ.keɴ]) is a prefecture of Japan located in the Chūgoku region of Honshu. Hiroshima Prefecture has a population of 2,811,410 (1 June 2019) and has a geographic area of 8,479 km (3,274 sq mi). Hiroshima Prefecture borders Okayama Prefecture to the east, Tottori Prefecture to the northeast, Shimane Prefecture to the north, and Yamaguchi Prefecture to the southwest. Hiroshima Prefecture also borders Ehime Prefecture for 74 metres (243 ft) on Hyōtanjima.

Hiroshima is the capital and largest city of Hiroshima Prefecture, and the largest city in the Chūgoku region, with other major cities including Fukuyama, Kure, and Higashihiroshima. Hiroshima Prefecture is located on the Seto Inland Sea across from the island of Shikoku, and is bounded to the north by the Chūgoku Mountains. Hiroshima Prefecture is one of the three prefectures of Japan with more than one UNESCO World Heritage Site.

Tokushima Prefecture (徳島県, Tokushima-ken; Japanese pronunciation: [to.kɯ̥ꜜ.ɕi.ma, tokɯ̥.ɕiꜜ.ma, to.kɯ̥.ɕi.maꜜ.keɴ]) is a prefecture of Japan located on the island of Shikoku. Tokushima Prefecture has a population of 682,439 (1 February 2025) and has a geographic area of 4,146 km (1,601 sq mi). Tokushima Prefecture borders Kagawa Prefecture to the north, Ehime Prefecture to the west, and Kōchi Prefecture to the southwest.

Tokushima is the capital and largest city of Tokushima Prefecture, with other major cities including Anan, Naruto, and Yoshinogawa. Tokushima Prefecture is located on the Kii Channel, connecting the Pacific Ocean and Seto Inland Sea, across from Wakayama Prefecture on the Kii Peninsula of the island of Honshu. Tokushima Prefecture is connected to Awaji Island across the Naruto Strait by the Ōnaruto Bridge as part of the Kobe-Awaji-Naruto Expressway, connecting the prefecture to the city of Kobe and the San'yō Expressway on Honshu.

The San'yō Region (山陽地方 San'yō-chihō) is an area in the south of Honshū, the main island of Japan. It consists of the southern part of the Chūgoku region, facing the Seto Inland Sea. The name San'yō means "southern, sunny (yō) side of the mountains" and contrasts with the San'in or "northern, shady (in) side of the mountains".

The region is generally considered to include the prefectures of Okayama, Hiroshima and Yamaguchi. Sometimes, the section of Hyōgo Prefecture that formerly comprised Harima Province is considered to be within the region as well.

Okayama Prefecture (岡山県, Okayama-ken; Japanese pronunciation: [o.kaꜜ.ja.ma, o.ka.ja.maꜜ.keɴ]) is a prefecture of Japan located in the Chūgoku region of Honshu. Okayama Prefecture has a population of 1,815165 (1 October 2025) and has a geographic area of 7,114 km (2,746 sq mi). Okayama Prefecture borders Tottori Prefecture to the north, Hyōgo Prefecture to the east, and Hiroshima Prefecture to the west.

Okayama is the capital and largest city of Okayama Prefecture, with other major cities including Kurashiki, Tsuyama, and Sōja. Okayama Prefecture's south is located on the Seto Inland Sea coast across from Kagawa Prefecture on the island of Shikoku, which are connected by the Great Seto Bridge, while the north is characterized by the Chūgoku Mountains.

Yamaguchi Prefecture (山口県, Yamaguchi-ken) is a prefecture of Japan located in the Chūgoku region of Honshu. Yamaguchi Prefecture has a population of 1,377,631 (1 February 2018) and has a geographic area of 6,112 km (2,359 sq mi). Yamaguchi Prefecture borders Shimane Prefecture to the north and Hiroshima Prefecture to the northeast.

Yamaguchi is the capital and Shimonoseki is the largest city of Yamaguchi Prefecture, with other major cities including Ube, Shūnan, and Iwakuni. Yamaguchi Prefecture is located at the western tip of Honshu with coastlines on the Sea of Japan and Seto Inland Sea, and separated from the island of Kyushu by the Kanmon Straits.

Awaji Island (淡路島, Awaji-shima) is an island in Hyōgo Prefecture, Japan, in the eastern part of the Seto Inland Sea between the islands of Honshū and Shikoku. The island has an area of 592.17 square kilometres (228.64 square miles). It is the largest island of the Seto Inland Sea.

As a transit between those two larger islands, Awaji originally means "the road to Awa", the historic province bordering the Shikoku side of the Naruto Strait, now part of Tokushima Prefecture.

The Akashi Kaikyo Bridge (Japanese: 明石海峡大橋, Hepburn: Akashi Kaikyō Ōhashi) is a suspension bridge that links the city of Kobe on the Japanese island of Honshu and Iwaya, Awaji [ja] on Awaji Island. It is part of the Kobe-Awaji-Naruto Expressway, and crosses the busy and turbulent Akashi Strait (Akashi Kaikyō in Japanese). It was completed in 1998, and at the time, was the longest central span of any suspension bridge in the world, at 1,991 metres (6,532 ft). Currently, it is the second-longest, behind the 1915 Çanakkale Bridge in Turkey that was opened in March 2022.

The Akashi Kaikyo Bridge is one of the key links of the Honshū–Shikoku Bridge Project, which created three routes across the Seto Inland Sea.

Itsukushima (厳島) is an island in the western part of the Inland Sea of Japan, located in the northwest of Hiroshima Bay. It is popularly known as Miyajima (宮島), which in Japanese means "Shrine Island". The island is one of Hayashi Gahō's Three Views of Japan specified in 1643. Itsukushima is part of the city of Hatsukaichi in Hiroshima Prefecture. The island was part of the former town of Miyajima before the 2005 merger with Hatsukaichi.

Itsukushima is famous for the Itsukushima Shrine, a UNESCO World Heritage Site. The shrine was considered a sacred site for a long time, it is possible that locals built a simple shrine on the site before the complex was built in 593 AD. In 1168 AD, Taira no Kiyomori, a warrior-courtier, greatly contributed to giving the shrine its current form. In the 16th century, Toyotomi Hideyoshi, a famed Japanese warlord, built a large building, the Senjō-kaku, on a hill above the shrine.

Setonaikai National Park (瀬戸内海国立公園, Setonaikai Kokuritsu Kōen) is a Japanese national park, comprising areas of Japan's Seto Inland Sea, and of ten bordering prefectures. Designated a national park in 1934, it has since been expanded several times. It contains about 3,000 islands, known as the Setouchi Islands, including the well-known Itsukushima. As the park encompasses many non-contiguous areas, and covers a tiny proportion of the Inland Sea's total extent, control and protection is problematic; much of the wider area is heavily industrialized.

The Kii Channel (紀伊水道, Kii-suidō), also called the Kii Strait, is a strait separating the Japanese island of Shikoku from the Kii Peninsula on the main island of Honshū. This strait connects the Inland Sea with the Pacific Ocean.

The name of the strait derives from Kii Province, a former province on the Kii Peninsula.

The Akashi Strait (明石海峡, Akashi Kaikyō) is a strait between the Japanese islands of Honshu and Awaji. The strait connects Seto Inland Sea and Osaka Bay. The width of the Akashi Strait is approximately 4 kilometers, and maximum depth is about 110 meters. The fastest tidal current is about 4.5 metres per second (8.7 knots).

The 1.5-kilometer strait is one of the important points of the Seto Inland Sea and is at the mouth of the Pacific Ocean. The surrounding waters around Akashi Strait is a known fishery area. The Akashi Strait is designated as an international shipping channel by the Maritime Traffic Safety Act in Japan.

Ōita Prefecture (大分県, Ōita-ken; Japanese pronunciation: [oꜜː.i.ta, oː-, oː.i.taꜜ.keɴ]) is a prefecture of Japan located on the island of Kyūshū. Ōita Prefecture has a population of 1,081,646 (1 February 2025) and has a geographic area of 6,340 km (2,450 sq mi)). Ōita Prefecture borders Fukuoka Prefecture to the northwest, Kumamoto Prefecture to the southwest, and Miyazaki Prefecture to the south.

Ōita is the capital and largest city of Ōita Prefecture, with other major cities including Beppu, Nakatsu, and Saiki. Ōita Prefecture is located in the northeast of Kyūshū on the Bungo Channel, connecting the Pacific Ocean and Seto Inland Sea, across from Ehime Prefecture on the island of Shikoku. Ōita Prefecture is famous for its hot springs and is a popular tourist destination in Japan for its onsen and ryokan, particularly in and around the city of Beppu. It has more onsen than any other Japanese prefecture.