Previous studies have reported that people with autism spectrum disorder (ASD) have higher levels of loneliness than neurotypical (NTP) people, most likely because of their difficulties in social communication with their predominantly NTP peers. However, direct investigations on the causal influence of friendship on their feelings of loneliness is scarce.In the current study, using the causal mediation analysis, we investigated whether friendship among ASD individuals influences their feelings of loneliness, especially during adolescence when the importance of friendship is typically most elevated. Furthermore, we examined whether individual differences in autistic behavioral features or age affect feelings of loneliness or the qualities of friendship with linear regression analyses.The results demonstrated that the higher levels of loneliness in adolescents with ASD were mediated by one aspect of friendship, companionship. We also found that positive aspects of friendship, but not negative aspects, influence the feelings of loneliness in both ASD and NTP populations. One subcategory of the measured autistic trait, difficulty in imagination, which is associated the ability to consider another's perspective, had a negative relationship with the positive aspects of friendship in the ASD group, but not in the NTP group.These findings indicate that the quality of the positive aspects of friendship is similarly important for both adolescents with ASD and NTP adolescents, but the autistic behavioral features could interfere with the experience of such positive friendships.