The main research question addressed in this chapter is: What is the significance of the emergence of corporate health insurance associations as health insurers in 1922? My hypothesis is that the Japanese government integrated major corporations in a corporatist framework established by the introduction of the Health Insurance Law. The enactment process of the Japanese Health Insurance system, initiated and led by the government, set up a corporatist structure encompassing big corporations and the Japan Medical Association. Corporations were granted financial, legal, managerial, and other incentives by the government, which at the same time imposed legal constraints on corporations to manage their behavior. This chapter focuses on how big businesses were incorporated into this framework.