This study examined the Theory of Mind (ToM) abilities of typically developing preschoolers in three age groups: three- to four-, four- to five- and five- to six-years-old (n = 110), who differed in their spontaneous social interest toward included children with autism spectrum disorder (ASD). Social interest was assessed by administering a teacher questionnaire and by nominations of the teacher and educational aide. ToM measures included five tasks of mental-state understanding: (1) diverse desires; (2) knowledge access; (3) diverse beliefs; (4) false belief; and (5) hidden emotions. ToM levels were significantly higher in the socially interested group only for the youngest age group of three- to four-years-old. In this age group, children who showed social interest toward the included peer had higher ToM scores than children who did not reach out to peers with ASD. For better understanding of social interest in the older two age groups, factors other than ToM should be defined and investigated further.