Being appropriately aware of the extent of stress experienced in daily life is essential in motivating stress management behaviours. Excessive stress underestimation obstructs this process, which is expected to exert adverse effects on health. We prospectively examined associations between stress underestimation and mental health outcomes in Japanese workers. Web-based surveys were conducted twice with an interval of 1 year on 2359 Japanese male workers. Participants were asked to complete survey items concerning stress underestimation, depressive symptoms, sickness absence, and antidepressant use. Multiple logistic regression analysis revealed that high baseline levels of 'overgeneralization of stress' and 'insensitivity to stress' were significantly associated with new-onset depressive symptoms (OR = 2.66 [95 % CI, 1.54-4.59], p < .01) and antidepressant use (OR = 4.91 [95 % CI, 1.22-19.74], p < .05), respectively, during the 1-year follow-up period. This study clearly demonstrated that stress underestimation, including stress insensitivity and the overgeneralization of stress, could exert adverse effects on mental health.