The legal fiction (fictio iuris) seems to be one of the most curious instruments of the Roman legal method. The author tries to find out the reasons which lead to the development of the fiction, analyzing the ancient formula of adoption (adrogatio). The formula declared the adoptee to be son of the adopter «as if he had been born of him in legal marriage». According to the author, the procedure was rooted in the Romans’ idea of the legal order. The potestary structure of family seemed to be an element of the objective normative structure of the society, which was not a product of legislation and could not be changed by an arbitrary normative prescription. The relations of patria potestas appeared to be inseparable from the legal institution of family as a whole and could exist only between father and his son conceived in legal marriage. Therefore, to accept a person into a family as a son, the Romans would resort to legal fiction: they did not just subdued him to the adopter’s patria potestas (which would mean a distortion of the original family structure), but transferred the whole of the institution on a new ground, declaring the adopted individual to be the adopter’s son as if he had been born of him. This technique made it possible to legalize nontypical patterns of social interaction in exceptional cases without changing the existing normative order.