The Seto Inland Sea is the largest enclosed coastal sea in Japan. It is enclosed by three of the four main islands of Japan: Honshu, Kyushu and Shikoku. The Seto Inland Sea covers an area of 23,000km2 and has a coastline that stretches for a total length of 6,900 km. It can hold 880 billion cubic meters of water and is a shallow sea with an average depth of 38 meters. The Seto Inland Sea is dotted with approximately one thousand small islands and is connected with the outer ocean via the Kii Channel between Honshu and Shikoku, the Bungo Channel between Shikoku and Kyushu, and the Sea of Hibiki between Honshu and Kyushu. The Seto Inland Sea has a temperate climate; the average temperature is 15℃ and the average rainfall is 1,000 - 1,600mm.

　The coastal regions bordering the Seto Inland Sea are home to 30 million people, 24% of the entire population of Japan (130million). These people are crowded into an area of 47,000km2 - a mere 12% of the entire land area of Japan (380,000km2).

　In terms of its climate and geographical conditions and the availability of a work force, the Seto Inland Sea is suitable as a location for heavy and chemical industries, particularly steel, and petrochemical industries, and the region played a leading role in helping Japan's period of explosive economic growth in the 1960s. For this reason, even today 50% of Japan's steel industries, 40% of its petrochemical industries, and 30% of its pulp industries are based in the littoral region of the Seto Inland Sea.

　Since ancient times, the Seto Inland Sea has played a leading role in maritime transport. About one half of all goods shipped domestically pass through the Seto Inland Sea, and numerous cargo vessels can be seen at any given time passing to and fro through the narrow sea.

　The Seto Inland Sea is also a major fishing area, with a yearly catch of 270,000 tons and a yearly cultivation of 320,000 tons. Compared to 1965, the fishing catch has not changed very much, but the amount of fish cultivation has doubled. It should also be noted that, as reclamation for the construction of industrial lands has reduced the number of fishing areas, the quantity of clams and other marine produce has fallen dramatically to one-fourth of its former levels.