The purpose of this study was to examine a young child's developmental change of utterances about causality concerning others, analyzing data videotaped at home in unstructured observations of relationship between a boy and his younger brother. At 30 months, the boy was able to talk about causality concerning his younger brother's behavior/action and physical reality. At 31 months, the boy was able to talk about causality concerning his younger brother's internal states, social practices, and abilities/capabilities. Through the observation period, the boy talked most frequently about his younger brother, and most of his talk was addressed to his younger brother. The data suggested that the boy's talk varied by the partners and by the contexts and that especially in the situations, his talk varied by time period and by his parents' existence or interventions. The findings suggested that the boy would talk about causality when he felt his younger brother's behavior was strange and immature.