The Mental Health Action Checklist for a Better Workplace Environment (MHACL) is a tool for a worker participatory approach to improve work environments for worker mental health. The present study investigated the effects of an organizational intervention using the MHACL on reducing job stressors and the psychological distress of workers of a manufacturing enterprise in Japan with a controlled study design. Nine of 45 departments participated in a work environment improvement program, including planning workshops, implementation and monitoring, between July and December 2005 (intervention group, n=321). The remaining 36 departments served as the control group (n=750). Outcomes (job stressors, worksite support, psychological distress, etc.), measured using the Brief Job Stress Questionnaire, as well as sick leave days taken from the company record, were recorded before and six months after the program for both groups. Among women, skill underutilization, supervisor and coworker support, psychological distress, and job satisfaction changed more favorably in the intervention group than in the control group (p<0.05). No significant favorable effect of the program was observed among men. Improvements in the outcomes were more prominent among departments with a 50% or higher rate of worker participation in the planning workshops and among departments with a 50% or higher rate of implemented vs. planned actions. A worker participatory organizational intervention using the MHACL seems effective for promoting mental health among Japanese white-collar women.