An interpretation is presented for the Cenozoic geological evolution of the offshore San' in and Kita-Kyushu areas around the western part of Japan on the basis of seismic survey. Along with well data obtained from the study area, geophysical data describe the rifting of the Sea of Japan in the early Miocene which caused rapid subsidence and wrench deformation in the San' in and Kita-Kyushu areas, respectively. As for the San' in area, it seems that stress regime changed from E-W to N-S compression in the late Miocene. Based on a quantitative basin analysis model, immature potential source rocks in the San' in area can be attributed to the reduced heat flow through the late Miocene.