Cretaceous volcanic rocks (volcanic, pyroclastic and lesser sedimentary rocks) are distributed in the Yanahara area, Okayama Prefecture, eastern Chugoku, SW Japan. Low gravity anomalies concordant with the thick Cretaceous volcanic rocks (Ishikawa et al.,2014) suggest a concealed caldera(cauldron), the infilling of which consists of siltstone, welderd tuff, crystal tuff, rhyolite lava, tuffaceous sandstone, andesite lava, lapillistone and lapilli tuff, in ascending order. The rhyolite lavas laterally attenuate and change to well-bedded rhyolitic lapillistones and tuffaceous sandstones. These volcanic, pyroclastic and epiclastic tuffaceous rocks, which gently incline northward in the main part, shows nearly vertical attitude along the cauldron margin. Namely, they form a half basin plunging north. Some ring dikes of quartz porphyry and andesitic porphyry are also found along the margin, and some fracture zones intercalating fault breccias are observed in the peripheral basement rocks. These structures suggest syn-eruptional subsidence along the boundary faults. The vertical displacement of the marginal fault is estimated more than 1 km by the borehole evidence (MMAJ 1980).