The central theme of Japanese subnational governmental study has been center–local relations. Researchers looked into the question of whether Japanese subnational governments have sufficient autonomy to pursue their own preferences. In other words, the policymaking process of subnational governments in Japan has long been examined in relation to the national government. It is true that, under the centralized governmental system, the national government of Japan exerts a substantial influence on subnational policymaking. Students of subnational governments cannot avoid this issue. However, one cannot understand the reality of subnational policymaking only by looking at the center–local relations and the administrative institutions that define the relations. Indeed, one needs to ask what determines subnational policy outcomes, to what degree the national influence affects the determinants, and under what conditions these determinants function autonomously. The present paper directly examines the subnational policymaking process itself, not the center–local relations, and tries to understand its mechanism. The paper proposes a research framework using a statistical method that has been developed in the area of diffusion study. In the framework, national influence is integrated into an analysis of subnational policymaking as one of the factors that affect policy determinants. Using the framework, the paper analyzes the process of four regional policies, which will lead to a better understanding of subnational policymaking.