We evaluated the effects of social monitoring and feeding competition on foraging success in relation to the feeding group size of wild Japanese macaques (Macaca fuscata). Social monitoring is visual scanning by group members that assists them in following their own group. Individuals in smaller feeding groups may frequently use social monitoring while foraging, because they have an increased risk of losing their group. Therefore, social monitoring could be a cost for group-foraging animals. We made four predictions: (1) individuals in smaller feeding groups tend to abandon food patches to follow group members; (2) social monitoring frequency is higher in smaller feeding groups; (3) feeding rate decreases with increased social monitoring frequency; and (4) feeding rate initially increases with feeding group size because decreased social monitoring outweighs increased feeding competition, but after the feeding group reaches a certain size, feeding rate declines with increasing feeding group size due to the high costs of feeding competition. These predictions were supported by our results. Thus, the relationship between feeding group size and feeding rate can show three patterns (positive, neutral and negative) in response to the balance between the costs of social monitoring and feeding competition.