Purpose: To verify the reliability and validity of the Assessment of Positive Occupation 15 (APO-15) in individuals with mental disabilities living in communities or admitted in hospitals. Methods: A sample of 408 individuals with mental disabilities completed APO-15, the Japanese version of the Recovery Assessment Scale (RAS), the Japanese version of the Self-identified Stage of Recovery Part-B (SISR-B), and the General Health Questionnaire 12 (GHQ-12). We analyzed the psychometric properties of APO-15, including confirmatory factor analysis, entropy, polyserial correlation coefficient, average variance extracted, Cronbach’s α coefficient, Pearson’s product–moment correlation coefficient, item response theory, and cut-off point. Results: This study indicated the validity and reliability of APO-15 in a group of individuals with mental disabilities. The result of this study supported a four-factor model constructing of 15 items; includes a positive relationship, achievement, meaning, and engagement. Validity was supported by various results, i.e. the polyserial correlation and entropy were good, confirmatory factor analysis was a good estimate of the model fit, hypothesis testing was good convergent and discriminant validity, and concurrent validity also good. In addition, reliability was established by various analyses, i.e. the internal consistency reliability was good, and all items of APO-15 demonstrated satisfactory item response. The cut-off point became a 42-point sensitivity (0.770) and demonstrated good results with 1-specificity (0.441). That is, APO-15 can be used to appropriately measure the participation in occupation to promote the well-being of clients. Conclusion: APO-15 demonstrated good psychometric properties in measuring positive occupation in individuals with mental disabilities. APO-15 is an important tool to enable participation in activities that increase well-being in daily living.