The present article investigated the longitudinal influence of loneliness on peer acceptance in school through two longitudinal studies; specifically, we hypothesized a bidirectional view on the relationship between loneliness and peer acceptance. In Study 1, a total of 383 Japanese elementary school students in fourth and fifth grades (207 boys and 176 girls, aged 9–11) at three public schools in Japan participated in an 18-month longitudinal study. Through a comparison of hypothetical models, the accepted model showed a ripple effect of loneliness on peer acceptance. Subsequently, in Study 2, a total of 506 Japanese elementary school students in fourth to sixth grades (253 boys and 253 girls, aged 9–12) at two public schools in Japan participated in a 6-month longitudinal study. The statistically accepted cross-lagged model indicated a bidirectional influence between loneliness and peer acceptance. These findings indicate a strong link between loneliness and peer acceptance from a longitudinal bidirectional perspective and suggest the importance of early educational practices for lonely children to prevent them from entering this vicious cycle.