Seto Inland Sea in the context of "Okayama Prefecture"

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.