In the ancient census register books, a componential unit group is a household. Ancient household can not be regarded as a family. They often include several families, boaders ("Yorikuchi, Kiko" "Yoriudo, Kijin"), and slaves ("Nu, Hi "). In this article, examining the ancient register books and commentary books of "Ritsuryo" ("Ryo no Gige", "Ryo no Shuge".), .1 assume the determinants of a household as follows ; . i) A household is not always composed of one individual family (nuclear family). ii) A household must include more than two male adults (above sixteen years old). iii) Only a patrilineal offspring can succeed to his father's household. iv) A household is avowedly administrative organization which is liable to taxation. v) The head of a household can control all of families within his household. Recently, on explanation of a household, professor Koji Hirata offered a very important and interesting hypothesis. He assumed that a nuclear family was the. original from of a household and that the structure of a household in the ancient census ' register books became increasingly complex as the registration repeated. Judging from five items mentiond above and family cycle theory, I conclue that Hirata's hypothesis is acceptable only when it is adopted to the family of procreation of a certain person within a household.. If we apply the family cycle theory which tells us a man belongs to two kinds of families, i. e. family of orientation and family of procreation, we can not assume that the ancient household is originally formed on the basis of a nuclear familiy.