Continental European countries and Japan are running far larger welfare states, particularly in terms of elderly care, than their Anglo-Saxon counterparts. One reason for this is the filial obligations of adult children toward their retired parents as mandated by family law. In continental Europe and Japan, the retreat of the state implies an increase in the burden incurred by the family, which encourages both liberals and paternalistic conservatives to support such welfare states. Japan is in the same camp as continental Europe because of its Civil Code of 1896, modeled on French law. However, the essential reason why Japan followed French law in the first place was that early modern (Tokugawa) family law before the Meiji Restoration of 1868 was consistent with French law in terms of filial obligations. This paper investigates the process under which filial support became a legal mandate in the eighteenth century. Early modern Japan transformed filial support as a norm into a legal mandate by setting the performance of filial obligations as a condition for property rights protection in the age of Japan’s first aging population in the eighteenth century.