This study categorized interpersonal conflicts in friendships between mothers rearing little children, so called “mum-friends” in Japan. It examined the relation between the typical types of conflicts and the personality traits of the respondents and their friends assessed by the Big Five Scale (Wada, 1996). An internet survey was conducted with 300 women rearing little children. From the result of Hayashi's Quantification Method Type Ⅲ and cluster analysis based on 205 respondents who reported conflicts with their mum-friends, we extracted four clusters of types of interpersonal conflicts. Conflicts concerning criticism of children's discipline and lack of common-sense were frequent in the friendship between mothers; these respondents rated their friends’ Conscientiousness and Agreeableness as relatively low. For conflicts related to socioeconomic disparity, the respondents’ self-reported Conscientiousness was evaluated lower than for the other types of conflict, and the friends’ Conscientiousness and Agreeableness were evaluated relatively high.