Common marmosets (Callithrix jacchus) are known to be cooperative breeders, which demonstrate similar social structure as humans. Despite the interesting links between humans and other cooperative breeders, the contributions of helpers to the breeding pair beyond infant rearing remain largely unclear. Therefore, in this study, we observed intragroup interactions of six monogamous breeding pairs of captive common marmosets with varying numbers of offspring for 117 hours using video recordings at the Primate Research Institute, Kyoto University. The study aimed to test the hypothesis that as the number of offspring (prospective helpers) increase, the frequency of social interactions between the breeding pairs will decrease. This hypothesis was developed based on the possibility that helpers may contribute to a breeding pair's mate guarding, which could then reduce the necessity of strong bonding between the breeding pair. Although the results obtained from this study did not support the hypothesis statistically, we found interesting possible effects of sex, age, and pregnancy on the social relationships of breeding pairs. These findings indicate that social factors other than the number of offspring may be affecting the results of the study. Hence we propose possible future research plans aiming to investigate factors impacting social relations in common marmosets, including but not limited to the number of helpers, while taking into account the limitations of the present study.