Abstract Defense styles have a close relationship with ego development and psychopathology. The purpose of the present paper was to examine the reliability and validity of the Japanese edition of the Defense Style Questionnaire (DSQ-40; Andrews et al. 1993). Six hundred and six Japanese university students (348 female) completed the DSQ-40 and Maudsley Personality Inventory (MPI). The neurotic tendency had positive correlations with immature and neurotic defense styles and a negative correlation with the mature defense style. The extroversion tendency had positive correlations with the mature and neurotic defense styles and a negative correlation with the immature defense style. There was a significant test–retest correlation for all items of the DSQ-40 (n = 284). The results show that the Japanese version of the DSQ-40 had concurrent validity with the MPI and a test–retest reliability of 2 months, although there was some doubt about the validity of the factor structure of the DSQ-40. Therefore, the DSQ-40 is a useful scale in Japan to measure ego development and ego function through defense styles.