Background: The promotion of family nursing by public health nurses (PHN) presupposes an accurate assessment of the family nursing support they already provide. However, as there is no assessment tool for this purpose, this study aimed to develop a scale to assess family nursing currently provided by PHN. Methods: We developed the Family Nursing Practice Scale (FNPS) for PHN based on the results of a previous study. The content validity of the FNPS was established through discussion with three other researchers. A pilot study was conducted to confirm face validity. To confirm reliability and validity, an anonymous, self-reported questionnaire was sent to PHN working in municipal offices. The statistical analyses included the Kaiser-Meyer-Olkin (KMO), Barlett’s Test of Sphericity, exploratory factor analysis (EFA), Cronbach’s alpha, correlation coefficient and t-test. Results: Seven hundred fifty-four PHN participated in this study. Cronbach’s alpha of FNPS was 0.94. The KMO measure was 0.948, and Bartlett’s Test of Sphericity was p < 0.01. Two factors together accounted for 63.2% of the variance in EFA. No items were excluded because of low loadings. Construct validity was confirmed through comparison with categories from a previous study. The correlation coefficient of FNPS and selected items of the Practice of Breastfeeding Support Scale was r = 0.56 (p < 0.01). The result of the t-test showed that the FNPS score of PNH who had received training in family nursing was significantly higher than that of PHN who had not (t = -2.0; p < 0.05). Conclusion: The reliability and validity of the FNPS were confirmed. The FNPS comprised 15 items and two factors. The score for “Active support for the family” was lower than “Support given with awareness of the family’s situation.” The findings of this study strongly suggested that the FNPS would be effective in clarifying the current state of family nursing provided by PHN and factors related to this activity and thus greatly assist the efforts of PHN to promote family nursing.