Aim: This study aimed to develop a Family Concordance Competency Scale for Family System Units (FCCS-Fa) for families with children having chronic disease, and to evaluate its reliability and validity. Methods: FCCS-Fa was developed by taking the following steps: (a) drafting based on the elements comprising concordance between healthcare professionals and families with patients suffering from chronic illness; (b) evaluation of face and content validity by an expert panel; and (c) re-examination of face and content validity by semi-structured interviews with 16 families. Criterionrelated validity was evaluated using the existing scale and construct validity was evaluated using exploratory factor analysis. Analysis of each FCCS-Fa evaluation item, internal consistency, and the 2-week test-retest reliability was also conducted. An anonymous self-reported questionnaire survey was conducted, targeting families with chronically ill children who were outpatients at three hospitals. Results: A total of 196 subjects were analyzed. As results of FCCS-Fa item analysis and exploratory factor analysis, a scale structure comprised of 17 evaluation items and three factors were adopted. In addition, a significant correlation with several existing scales was identified and the criterionrelated validity was also confirmed. The Cronbach's α coefficient for the overall scale was .927, the intraclass correlation coefficient applying the retest method was .905, and internal consistency and test-retest reliability were both confirmed. Conclusions: We developed FCCS-Fa with reliability and validity. Assessing family concordance competency using this scale and supporting families to achieve family concordance can lead to self-management by families.