The koseki 戸籍 (family or household registry) has long served as a material representation of the conceptual structure of Japanese family relations. Membership in a family has been stipulated and proved through registration in a koseki document defined through a shared surname and address. Evidence of family membership for purposes of legal transactions and social interactions has rested in the koseki document. However, during the past several decades some women have questioned the social pressure and legal requirement to change their names in marriage, choosing instead to maintain their surname by refusing to register their marriages to their “husbands.” Claiming themselves “married” but not legally registering their marriages, this growing group of name-change resisters defines their nonregistered marriages as jijitsukon 事実婚 (common-law or real marriage). Drawing on ethnographic research with women in jijitsukon marriages in Tokyo who refuse to share a koseki with their “husbands,” this article explores the implications of marital registration resistance in a marriage-centric society and the concurrent critique of the koseki system (the Koseki Law, koseki document, and the broader system of registration) and the legal marriage structure at the core of women's claims to be married when they do not meet Japan's legal criteria for marriage.