Although social network service (SNS) is an important service to develop and maintain intimate relationships among university students, maladaptive behaviors like excessive use may result in serious psychological problems. In this study, we investigated the relationship between SNS use behaviors and attachment style that has been regarded as a reliable developmental origin of interpersonal problems and addiction. Especially, the relationship between anxious attachment and SNS addiction proneness was examined and the median effect of excessive reassurance-seeking on the relationship between the two variables was tested. We administered self-reports that assess SNS addiction proneness, excessive reassurance-seeking and anxious attachment style to a sample of university students (N = 271), and conducted correlation and mediation analyses. Our results showed that anxious attachment, excessive reassurance-seeking and SNS addiction proneness were positively correlated. In addition, it was found that excessive reassurance-seeking partially mediate the relationship between anxious attachment and SNS addiction proneness. These findings suggest that it may be crucial to consider interpersonal features such as anxious attachment and excessive reassurance-seeking when treating SNS addiction proneness in university students.