Objective: To study the improvement of the mental health of Japanese male registered nurses, we investigated the predictors associated with the Japanese version of the 12-item General Health Questionnaire (GHQ-12). For those predictors, we focused on environments with many female registered nurses and female patients’ refusal to accept nursing services from male registered nurses.Materials and Methods: This study was conducted in 93 hospitals in the Aichi, Gifu, and Mie prefectures of Japan. The analyzed subjects were 1,216 full-time male registered nurses. The average age (standard deviation) was 34.5 (9.2) years. Multiple logistic regression analysis was conducted. A GHQ-12 score of ≤3 indicated good mental health, and a score of ≥4 indicated poor mental health. “Mental health” was a dependent variable. The independent variables were: “Age”, “Male registered nurses as important partners on the job”, “Female registered nurses’ good manners toward male registered nurses”, “Fairness regarding male registered nurses’ promotions”, and “Female patients’ refusal to accept nursing services from male registered nurses”.Results: There were 728 (59.9%) full-time male registered nurses with good mental health and 488 (40.1%) with poor mental health. The mental health of the male registered nurses who had negative feelings regarding, “Male registered nurses as important partners on the job” was significantly worse, and that of those who had negative feelings regarding, “Female registered nurses’ good manners toward male registered nurses” was also significantly worse.Conclusion: Female registered nurses must recognize that their attitudes toward male registered nurses influence the mental health of male registered nurses. Hospital managers should provide male registered nurses who cannot establish appropriate relations with female registered nurses with consultation opportunities. Such organizational action by managers is necessary.