In Chapter 4, the idea of “illegitimacy” and its formidable role in the koseki system are examined. The author traces attitudes toward male sexuality and kinship from the Meiji period to the present to better illuminate the koseki categories of “illegitimacy” (hichakushitsuko) and acknowledgment (ninchi) that play a central role in identifying children on the koseki. The unequal status of men and women as parents can be seen in the Civil Code rules that give men the right to acknowledge or refuse to acknowledge children born to their non-marital sex partners. The koseki’s function as an arbiter of lineage and legitimacy becomes clear through a number of case studies and historical analyses. In this chapter, the author explores the significance of the categories of the acknowledged child (ninchi sareta ko) and the “illegitimate” child to better understand deep assumptions about sexual and gender norms that seem to be the warp and weft of Japanese patriarchy.