We investigated the relationships between Autism-spectrum Quotient (AQ) scores, adult attachment style (as assessed by the Internal Working Model [IWM] scale) scores, and social skills (as assessed by Kikuchi’s Scale of Social Skills [KiSS-18]) in university students who had no diagnosis of autism spectrum disorder (N = 468). The AQ consists of five subscales: social skills, local details, attention switching, communication, and imagination. The IWM is composed of three subscales: secure, anxious, and avoidant attachment styles. The KiSS-18 is a single-factor model. First, we calculated the correlations between AQ, IWM, and KiSS-18 scores. Next, we examined the differences in each subscale score of the IWM between two groups defined by their AQ scores (High and Low AQ groups). We found that the High AQ had higher scores on the IWM secure subscale than did the Low AQ group. In addition, the High AQ group had lower scores on the IWM anxious and avoidant subscales than did the Low AQ group. Moreover, in the High AQ group, the secure style, but not the anxious and avoidant styles, modulated the KiSS-18 scores. The results of the present study add to existing knowledge of the relationships between autism spectrum tendency, adult attachment style, and social skills, and suggested that adult attachment styles (particularly the secure style) may play the role of mediator of social skill ability.