The authors have been studied the stratigraphy and geological structures of the late Miocene-Pliocene Ashigara group on the northwestern part of Kanagawa Prefecture in central Japan. In the process of the study, the authors found out a small volcanic collapse basin, and named the Yamakita group for the strata burying the basin. The Yamakita group is nearly flat in its structure and abuts to the high angled plane of the unconformity on its base. The marginal basal part of the Yamakita group consists of the huge angular boulder bearing ill-sorted conglomerate is developed, and the upper part of the group is composed of the andesitic pyroclastic rock. From the fact that the Yamakita group is surrounded by the high angle plane of unconformity which links up with the faults with the indurated fault breccia or clay in the Ashigara group, and that the volcanic activity concentrates in the basin of the Yamakita group, the formation process of the basin is concluded as follows: Firstly the expansion of magma resevoir caused the domal uplift of surfacial crust in part where horizontal tensile stress was generated to form many high angled normal faults. Then, the central area of the domal uplift collapsed down to form a basin and the talus deposits filled up a part of the collapse basin. Soon the volcanic activity broke out in this basin and the volcanic materials filled up a part of the basin.