Background Given the growing role of family caregivers in meeting the increasing demand for ageing-related care in Japan, the occupational adaptation of family caregivers needs to be evaluated. Methods This study developed the Occupational Adaptation Questionnaire for Family Caregivers (OAQC) and evaluated its reliability, validity, and effectiveness. To develop the OAQC item pool, the construct ‘Occupation of nursing care engaged in by family caregivers’ was evaluated. Using the Delphi method, 8 experts evaluated 64 items and confirmed that 41 items adequately represented the domain. The scale was then completed by 216 family caregivers of users of outpatient service establishments affiliated with hospitals in the Chubu region. The validity of the scale items was tested, and statistical analysis was performed. The construct validity and internal consistency of the OAQC were examined using exploratory factor analysis and the ω coefficient, respectively. Results The discrimination ability and difficulty level were calculated using the item response theory. The findings showed that the scale’s reliability and validity were satisfactory. Construct validity was acceptable for 16 items with 5 factors. The OAQC also had high internal consistency, reliability, and effectiveness. The item slope parameters and difficulty parameters revealed good item response, indicating that the scale could effectively measure family caregivers’ occupational adaptation. Conclusion The scale was compatible with the data obtained from family caregivers. Given its overall effectiveness, data obtained using the OAQC can contribute towards implementing personal care support programs for caregivers and enriching their lives by offering support from an occupational adaptation perspective.