Mentalization is an important ability to acquire for children, as it allows humans to understand the mental state of others or oneself, that underlies overt behavior (Fonagy & Target, 1996). In the current study we examined the relationship between development of mentalization ability in children and their experience of playing with a doll by observing child-mother interaction and by using functional near-infrared spectroscopy (fNIRS). 44 dyads of children aged 2 to 3 and their mothers were divided into two groups (high and low) depending on the frequency of doll-play experience. We examined mother-speech interaction during the doll play. We also used fNIRS system to measure cerebral hemodynamic activation in the frontal and temporal regions during the observation of video clips showing hindering and helping behaviors. The results showed that a mother's proxy talk was related to a child's doll directed speech in the high group, but not in the low group. fNRIS data showed that cerebral activation in the helping condition was more increased in the low group than the high group. This suggests that doll-play experience facilitates the development of mentalization, which enables children to be aware of and understand other's psychological states.