Supervisors at work play a large role in stress management at the workplace. Providing supervisors with necessary information and useful skills might be one effective approach that will lead to stress reduction. However, very few studies have investigated the effect of supervisor education by using a rigorous study design. In a randomized controlled trial, we tried to clarify how an education program for stress reduction influences supervisor knowledge, attitudes, and behavior concerning stress management. The subjects were 46 supervisors of an old, established sake brewery manufacturer of 301 employees. The supervisors were assigned to either the intervention group (24 supervisors) or the control group (22 supervisors). We conducted a single-session education program that included the guidelines for worker mental health promotion to the intervention group. The education program was composed of a basic education lecture and active listening training. The effects of this program on supervisor knowledge, attitudes, and behavior were measured using an original, self-administered questionnaire. The intervention effect was tested by examining an interaction effect between groups and time (before education, three and six months after education). The education favorably affected supervisor knowledge (F=7.92; p=.001). As for behavior, the intervention effect was marginally statistically significant (F=2.51; p=.088). For the attitude score, however, there were no beneficial effects. In conclusion, the provision of necessary information and useful skills to supervisors seems to improve supervisor knowledge and behavior regarding stress management at the workplace for at least six months.