The aim of this study was to elucidate the influence of the mothers' intervention styles in the sibling interaction at home on the social behavior that her firstborn child shows in the kindergarten. In a questionnaire, mothers answered that they mostly praise the firstborn child when he/she interacts friendly with younger sibling, and they tend to scold the firstborn child when he/she interacts agonistically with younger sibling, or mothers inform the firstborn child of the feelings of the younger child. Mothers tended to intervene in their girls than boys in the sibling interaction. Ten mothers were classified into three groups according to their intervention styles in their children's interaction at home, first-child intervention group (5 mothers), second-child intervention group (3 mothers), and equally intervention group (2 mothers). The firstborn children of the second-child intervention group showed more social behaviors than the children of the other two groups.