In order to evaluate simply the Japanese-type river water quality, a practicable method using differentiating diatom groups is proposed. Japanese rivers are generally shorter, shallower, and run faster. Therefore, though the present grouping is considered to be equivalent to Lange-Bertalot's (Category 1-3, Nova Hedwigia Beih. 64: 285-304. 1979), considerable differences have resulted in the members of the relevant groups. For the evaluation of the water quality of rivers, the employment of the formula of Pantle-Buck giving a saprobic value of 4 to the Group A taxa, 2.5 to the Group B taxa, and 1 to the others is suggested. This method is simple and easy and the results of applying this method to the Japanese river waters were in general agreement with chemical analyses.