In this paper, Inoceramus japonicus Nagao et Matsumoto, 1940 is discriminated into I. (Platyceramus) higoensis sp. nov., I. (Pl.) japonicus hokkaidoensis su bsp. nov., and I. (Pl.) japonicus japonicus, which approximately correspond to forma α, β and γ of the original description, respectively. And I. (Pl.) higoensis is subdivided into I. (Pl.) higoensis higoensis and I. (Pl.) higoensis su bsp. At first I mention the procedures of the specific and the su bspecific discrimination at population level. Some selected characters are biometrically and statistically examined by means of the Student's t-test, the rejection ellipse, 75 percent rule and analysis of the relative growth. Then I give the specific and subspecific definition of the above taxa, comparing them with already known species. Finally I discuss the phylogenetic relationships of the concerned species and subspecies.