It has been considerd generally that the sexual offences are committed by the mental deficiency. The present study was desingned to investigate their actual circumstances, in order to treat such delinquents adequately.

The subjects in this study were 51 feeble minded boys who had committed sexual offences and have been detained in the Tokyo Medical Reform and Training School. They were divided into five types as follows: (1) rape by turns, (2) rape on adults by himself (victims were over 18 years old), (3) rape on girls by himself (victims were from 14 to 17 years old), (4) rape on children (victims were under 13 years old), (5) sexual deviations.

There is remarkable differences between “rape by turns” and “rape by himself” group. “Rape by turns” group have not committed so many delinquent behavior, and they are apt to do so-called “socialized delinquent behavior”. Their intelligence are rather high, and their physical functions are active. They have not so remarkable deviation in their charcter, and not so abnormal interest in the female, and they can cooperate with other people.

On the other hand, “rape by himself” group have many deviant characteristics. Their emotion and physical functions are immature. They have much interest in the female, but they could not express it in the moderate way. This tendency was remarkable in the “rape on children” and “sexual deviatidns” group.

Many people believe that most of sexual offences have good prognosis, but we must consider their type and content, and must treat them.