In studying social structure, first of all, the social anthropologists pay attentions to family and kinship organization. In a sense, family is a kind of kinship organization characterized by its common residence and household. For the social anthropologists, though the concept of kinship is the most important clue to analyze social structure, the definition of kinship is very ambiguous. Kinship is ordinarily defined by blood relation and affinity, and for that reason we are apt to understand it either through a physiological aspect or a legal aspect. Is it relevant that we, the social anthropologists, examine kinship relations only through physiological and legal aspect? When we think about kinship relations, we must give attention to the relation between kinship relations and social norms, institution of marriage, rule of descent, rule of residence, and so on. When we describe kinship relations, we deal not only with a scheme of inter-personal relations, but also with social behaviors of the people related to each other by kinship tie. If we want to understand kinship relations thoroughly, we must, therefore, take an analytical point of view as follows ; the kinship relation is not only physiological and legal fact but a social relation.