Sedimentary rocks, mostly shale and sandstone, are exposed sporadically along the southern margin of the late Mesozoic ignimbrites distributed widely in Hiroshima Prefecture. The stratigraphic relationship between the two has not been found as yet. Tashima is an island located south of the ignimbrites, and sedimentary formations composed of sandstone and shale are exposed along the southern seashore of the island. Ohbatake formation (over 250 m in total thickness) is one of them and has been thought to be Palaeozoic in age because the pelitic rocks in this formation have phyllitic or gneissose structure. Through the present study, however, this so-called Palezoic formation is found to be overlain conformably by the late Mesozoic ignimbrites and therefore to be Cretaceous in age. The ages of similar sedimentary rocks in the Seto Inland Sea and in its vicinity may have to be reexamined in the light of this study.