Background: Chronic mental stress has been reported to be directly or inversely proportional to blood pressure (BP). To explain this inconsistent relationship, we assumed effect-modification by body mass index (BMI). Methods and Results: We examined 1,673 Japanese male local government employees who were not taking antihypertensive drugs or had no history of cardiovascular disease. BP and BMI were recorded at yearly health checkups. Exposure to mental stress, smoking, drinking, exercising, and salty taste were checked by questionnaire in 1997 and 2002. The main effect and interaction of stress and BMI on the averages and changes of resting systolic and diastolic BPs over the 5 years were assessed by a general linear model by adjusting for confounders. Obesity (BMI ≥25kg/m²) was significantly related with higher average systolic and diastolic BPs (P<0.001, P<0.001, respectively), whereas mental stress was not, showing a significantly different relationship dependent on BMI (P for interaction =0.002, 0.004): a significant and directly proportional association with systolic and diastolic BPs (P=0.001, 0.001) in the obese, but borderline significant and inversely proportional association (P=0.07, 0.08) in the lean. Only BMI was significantly related to the degree of BP change. Conclusions: Whereas BMI was proportionally associated with BP, BMI was a modifier which, depending on its level, inverted the direction of the association between chronic mental stress and resting BP.  (Circ J 2014; 78: 1379–1386)