Interoception, that is, the perception of visceral stimuli, is the basis of socio-emotional development. However, no studies have demonstrated the relationship between the two in infants. This study aimed to elucidate the relationship between interoception and social behavior in infants and mothers. Visual preference for cardio-visual synchronous and asynchronous stimuli was assessed using a preferential-looking paradigm in 6-month infants and their mothers. The infant–mother interaction was also measured to assess social behavior, such as eye contact and positive facial expressions. The results showed that infants looked at asynchronous cardio-visual stimuli longer than synchronous cardio-visual stimuli, whereas mothers looked at synchronous cardio-visual stimuli longer than asynchronous cardio-visual stimuli. The proportion of looking time toward asynchronous cardio-visual stimuli in infants was positively correlated with infant–mother gaze and affect (positive facial expression) synchrony. Furthermore, mediation analyses showed that the relationship between infants’ interoception and eye contact behavior is attributable to mother’s positive facial expression. Our findings suggest that in infant–mother interactions, infants’ interoception may play a role in eye contact behavior through the mother's positive facial expression, highlighting the importance of infants’ interoception on social cognitive development.