Japan's immunization policy is often perceived as lagging behind those of other developed nations because of the delay in vaccine licensing and exclusion from the national program of some vaccines widely used elsewhere. In Japan, municipal authorities provide financial support for voluntary vaccinations, which are not included in the national program. This study examines the process of vaccination policymaking by municipal governments, focusing on the interdependency of such policy and using the spatial lag model and data from 2010. We make the following three findings. First, there are no systematic priorities on vaccines across municipalities. Second, vaccination subsidy policy is statistically significantly correlated with neighboring municipalities in the same prefecture, but not outside, indicating that Japanese municipalities engage in "yardstick competition" in the same prefecture. Third, no strong correlations between the other socio-economic or fiscal characteristics of municipalities and vaccination subsidy policy are detected.