This sedimentary basin comprises an area of about 35, 000 square kilometers, elongated in noth-south direction, mainly from Soya to Hidaka region in the middle part of Hokkaido Island, and with major basin-axis trending norht-south for approximately 390 kilometers. (Fig. 8) The basin now embraces some small oil and gas fields which produce from Neogene Tertiary sediments. The oil-bearing Neogene formations in the basin lie unconformable on the Paleogene, Cretaceous, and pre-Cretaceous or so-called Paleozoic formations. The following is a generalized standard section of the Neogene Tertiary sediments of this basin, in descending order. Holocene series……Alluvial deposite Pleistocene series……Terrace deposite Numakawa formation Pliocene series……Takikawa formation Oiwake formationMiocene series……Wakkanai formation Kawabata formation Chikubetsu formation Haboro formation Magaribuchi formaIion In this paper, Neogene formations are described, and their sedimentary environments are discussed on the basis of the smaller foraminfera contained. The present work was started on ten years ago, and geological observation and sampe collections were made during 1948 to 1953, when field works was undertaken for the purpose of petroleum exploration of the Teikoku Oil Company. The results of this work is concluded by the auther that the Neogene formations are sediments in the age of the Miocene Osillatory Transgression, and its highest tide in the basin-axis region or most broad spread of marine water in the basin occurred in the middle Kawabata stage (Fig. 10). It is regarded that the oldest horizon or Magaribuchi formation belongs to the sediments of middle Miocene stage and the lower Miocene sediments are not present in this area.