In a series of papers in 3 parts some new and little known inoceramid species from the Cenomanian of Japan are to be described. In this part we describe the following four species including three new ones. (1) Birostrina sp. nov., which belongs to the group of B. concentrica but has a radial groove on the posterior part and ribs of moderate intensity in the late growth-stage. It is represented by a number of specimens from the Middle Cenomanian of the Mifune (central Kyushu), Mikasa (central Hokkaido) and Kotanbetsu (northwestern Hokkaido) areas. (2) Inocermus sp. nov., which is allied to I. comancheanus but has a subrectangular outline with less convex valves and weaker subcostae. Its holotype came from the Lower Cenomanian of the Mikasa area. (3) I. heinzi Sornay, which is represented by several specimens from the Upper Cenomanian and/or lowest part of the Lower Turonian of the Saku area (northwestern Hokkaido). They are very similar to the holotype from Madagascar. (4) Mytiloides sp. nov., which has a larger beak angle and more convex anterior margin than M. submytiloides and is characterized by regular concentric ribs combined with fine concentric lirae. Its holotype came from the Zone of Euomphaloceras septemseriatum (Upper Cenomanian) of the Mikasa area.