The present study aimed to investigate whether coincidence of opinion affects the evaluative processing of outcomes in group decision-making under authority rule. For this purpose, we examined the effects of the opinion coincidence on feedback-related negativity (FRN), an event-related brain potential (ERP) reflecting the evaluative processing of outcomes. Six three-person groups performed a group decision-making task in which one member acting as a leader (leader blocks) made a group decision to choose one of two cards after he/she observed opinions of the other members acting as followers (follower blocks), and monetary gain or loss was contingent on the group decision. To examine the effect of the opinion coincidence, each trial of each individual was classified into one of three trial types: unanimous, majority, or minority trials. As a result, the amplitude of FRN was smaller for unanimous trials than for majority trials in the leader blocks. In addition, the amplitude of FRN was larger for majority trials compared to minority trials in the follower blocks. These results suggest that the coincidence of opinion in group decision-making affects the evaluative processing of outcomes, and this occurs even when roles and responsibilities over outcomes is explicitly clarified under the authority rule.