Stress estimation is involved in the production of stress management behavior. Stress underestimation could hinder efforts to adequately manage stress. This study developed a measure to assess stress underestimation beliefs. Questionnaire surveys were conducted by mail twice on 2,340 adults aged from 20 to 65 years. The data from the surveys were subject to factor analysis, from which we developed a 12-item scale to assess stress underestimation beliefs (SUB). The SUB included four subscales: “Excessive self-efficacy for managing stress,” “Insensitivity to stress,” “Overgeneralization of stress,” and “Evasive attitude towards stress.” The SUB had adequate internal consistency. Higher SUB scores were associated with being in the precontemplation stage of change in terms of stress management behaviors, working long overtime hours (more than 81 hours per month), and having fewer self-reported stress symptoms. The SUB is potentially useful for assessing stress underestimation in the context of stress management.