Focus groups (FGs) led by trial consultants are popular in the USA but not in Japan. Additionally, the effectiveness of this method has not been examined. This study examined the effect of FGs on the Saiban-in system. First, eleven undergraduates participated in FGs, discussing the perception of certain words (e.g., self-defense) and a theme (e.g., how to evaluate a wrongful act when losing self-control because of fear) that were points of dispute in a simulated case. The contents were compared with three law students’ estimation of how undergraduates perceived these topics. Second, a law student wrote a final case argument before and after reading a summary produced by FGs. Third, another set of thirty-one undergraduates participated in one of two conditions (whether based on an FG result or not), read the arguments (sixteen read arguments not based on an FG result; fifteen read arguments based on one), judged the case, reported their confidence in the judgment, and marked the words that affected them. The effect of the FG on the conviction rate was not significant. However, confidence in the not-guilty verdict increased and participants were influenced by the final arguments based on the FG result. This indicates the efficacy of focus groups in writing a final argument mentioned in the deliberation.