The name “Aritagawa. graben” was newly given to the area which is occupied by the Cretaceous system along the medial zone of the Chichibu terrain in the western Kii Peninsula. The Aritagawa graben is bounded on the east by a normal fault (Itao fault) trending N-S and dipping to the west ; to the east of it, the Cretaceous system is not distributed. The easternmost part of the Aritagawa graben including the Itao fault was re-examined. On the north and the south of the graben are exposed the basement Paleozoic Chichibu. complex and the Upper Jurassic Idani formation, respectively ; the latter may belong either to the Chichibu terrain or the Shimanto terrain. The Cretaceous system in the graben, consisting exclusively of the Futakawa formation (late Urakawanearly Hetonaian epoch in age), forms several anticlines and synclines (major folds), each of which is marked with the abbreviated symbol and is described. These folds are not crumpled at their axial parts irrespective of the angle between the northern and southern limbs and have, in most cases, axes plunging more or less to the west. Each stratum of the Futakawa formation displays, on this account, the arched pattern convex to the east (in the case of a syncline) or to the west (in the case of an anticline), on the geological map. Furthermore, on the eastern extension of one of the anticlines (A-VIII), a small block consisting of the Chichibu complex is exposed within the graben (hereafter, called simply as the “small block”). Faults which demarcate the “small block” on the north and the south converge to one another into the eastern end of the A-VIII axis. The northern boundary fault, without any marked sheared zone along it, trends NE-SW and dips gently (40°±) to the north. On the contrary, the southern one, trending E-W and standing essentially vertically, is represented not by a single fault but by several ill-continuous faults. Lenticular bodies, about 20 m in width, of the Chichibu complex and of the Futakawa formation crop out repeatedly between these faults. Both the northern and the southern boundary fault are formed at or near the basal part of the Futakawa formation. Each stratum of the Futakawa formation proximal to the faults takes the attitude essentially parallel to that of the fault planes, whereas the Chichibu complex in the “small block” forms a homoclinal structure trending approximately E-W. These facts observed in and near the “small block” lead to the following conclusions (1) In all probability, the major folding structures of the Futakawa formation within the graben reflect the superficial relief of the underlying basement. Namely, they were formed as a result of the differential vertical movement of the blocks into which the basement had been separated by the closely spaced small faults. (2) After the folding structures had been accomplished, another block movement took place in the basement. In the easternmost part of the graben, each block tilted more or less to the west. The eastern half of the Futakawa formation which had participated in the anticline of the A-VIII was eroded away and the basement became to crop out to form the present “small block”. The northern boundary fault of the “small block” might be regarded as a sort of the plane of slip which occurred near the basal part of the Futakawa formation. The southern one is probably the fault which originally bounded the block of the basement under the A-VIII.