The Japanese are said to exhibit fundamental differences in their interpersonal attitudes and interpersonal behaviour, according to whether they are interacting as members of an 'in-group', or whether they are interacting with members of an 'out-group'. Markus and Kitayama (1991) advocate the concept of self based on independence, and the concept of self based on interdependence, as conceptual tools with which to account for cultural differences and individual differences in social behavior. The object of this study is to examine how the dominant concept of the self among the Japanese differs with respect to situations involving 'in-groups' and 'out-groups'. In Study 1,we have integrated and refined various existing scales of measurement which determine the type of dominant concept of self (Kitayama et ai., 1991; Morikawa, 1992; Takata, 1992), and have devised a scale for analysing the dominant concept of self, according as it operates in situations involving 'in-groups' and in situations involving 'out-groups'. In Study 2,we set up as a hypothesis an interpersonal conflict situation relating respectively to 'in-groups' and 'out-groups' and investigate the efficacy of the dominant concept of self which influences the evaluation of strategies for coping with the conflict. Generally speaking, in 'in-group' situations, an attitude of compromise is favoured, while in 'out-group' situations, an attitude of 'doing one's own thing'. This tendency was strikingly shown in the category of the concept of self based on interdependence. In addition, we examined the kind of differences that can be found in the concept of self among the Japanese as this affects 'in-group' situations and 'out-group' situations. While the difference of situation did not have influence on the concept of self based on independence, the concept of self based on interdependence showed a striking dominance in situations involving 'in-group', compared with situations involving 'out-groups'.