The sedimentation of the base, litho-facies near the base, the key bed and the sedimentary environment of the formation, were studied on the Kurotaki Formation, which is distributed from the Minato river-section in the west, to the Obitsu river-section in the east, over the Boso Peninsula.<BR>1) The Kurotaki Formation lies on the Anno Formation unconformably at most places, for instance, the Takamizo-section, northern Mitsuishi mountain-section where the basal part of the formation varies its litho-facies and contains fossiliferous and impure limestone.<BR>2) Two types of litho-facies were recognized in the formation except at the basal conglomerate. One is the silty sandstone facies intercalated many tephras, the other is the massive tuffaceous sandstone facies that is very rare of tephra. Both facies are rhythmically repeated from the lower horizon to the upper one and useful for the observation of the correlation of the formation in the area.<BR>3) 2 set of tephras which had hitherto been known as Kd 23 (_??_TM 1) and Kd 38 (_??_TM 3) in the area were investigated. As the result, it is clear that the former does not continue at a few sections, but the later continues roughly.<BR>4) Except above 2 tephras, 5 new tephras, namely TM 2, TM 4-7 in descending order were found. Among them, TM 2 consisting of coarse-grained scoria is most important as a key bed because this tephra ranges horizontally so long all over the area.<BR>5) The beginning of the deposition of the formation in this area was roughly the same period. But more exactly speaking, from the distribution of new tephra TM 4-7, the sedimentation of the basal part of the formation was formed in different time.Therefore it is assumed that the Kurotaki transgression might be brought to fill up the depressed ground which had been formed by folding before the Kurotaki transgression.<BR>6) Judging from the tephra of TM 3-7, the Mitsuishi mountain area was the land until the beginning of the deposition of TM 3.<BR>7) After the deposition of TM 3, the area surrounding the western and eastern sides of the Sasa river-section was deposited in the deeper sea than the Mitsuishi mountain area was.<BR>8) We knew by the litho-facies that the eastern part of the surveyed area was deeper in the sea than the west area, when TM 1, 2 were deposited.