In this Journal (Vol.8, No.2, Vol.9, No.1) we have discussed the etiology and classification of mental deficiency, and found that several etiological classifications have been advocated. The one most frequently used is that of A. A. Strauss, who divides mental deficiency into the exogenous or brain injured group and the endogenous or familiar group. This classification has, however, imperfections regarding classification-validity and other procedural defects. For the purpose of educational diagnosis of mental deficiency, we have tried to set up the typological approach from the standpoint of behavior patterns and of personality traits. With relation to our approach, several problems about the procedural methodology were considered.