Pterostichus thunbergi Morawitz and its related species of Japan are revised based on the structure of membranous parts of their genitalia for the first time, under the name of the thunbergi species group. This species group is monophyletic and is placed in the subgenus Morphnosoma Lutshnik. The monotypic subgenus Moritapterus Berlov, erected for P. thunbergi, is synonymized with Morphnosoma. Comparative study of the endophallus revealed that P. thunbergi michinoku Nakane is a junior synonym of P. habui and that four species should be recognized in this group. Three of them are sympatric with P. thunbergi and one of them, Pterostichus (Morphnosoma) robustistylis, is described as a new species. Molecular phylogeny based on the COI gene (mitochondrial [mt]DNA) supported its independence as a distinct species. The resultant trees of mtDNA and comparative study of morphology revealed that P. thunbergi is probably non-monophyletic. In the present study we suggest the presence of further cryptic species of Morphnosoma in Japan.