This study aimed to clarify the development of conversational style in Japanese mother–child interactions. We focused on the frequency of speech overlap as an index of Japanese conversational style, with particular attention to ne, a particle produced by the speaker, and to backchannels, such as ‘uh-huh’, produced by the listener that support sympathetic conversation. The results of longitudinal observations of two Japanese mother–child dyads from approximately 0;11 to 3;3 suggest that an adultlike conversational style with frequent overlaps emerges in Japanese child-directed speech around the two-word utterance period, and a child's development of ne use is closely related to this shift.