A large amount of ironware from the end of the Yayoi period were excavated from sites along the coastal areas of the Genkai-nada Sea, but very little were excavated from sites along the coastal areas of the Seto Inland Sea and Kinki region.
弥生時代後期には、玄界灘沿岸地域の遺跡から鉄器が大量に出てくるが、瀬戸内海沿岸各地方や近畿地方の遺跡からはごくわずかしか出てこない。