This work proposes a new institutional design for cultivating preventive-justice professionals in Japan, grounded in historical analysis and international comparison. Preventive justice refers to early identification of risks, cross-sector coordination, and practical legal intervention in schools, welfare agencies, and local governments—areas where Japan currently lacks a structured professional pathway. The paper clarifies three persistent misconceptions in Japanese policy debates: (1) the old bar examination was historically and internationally abnormal in difficulty; (2) Japanese legal practice prioritizes accuracy and precedent adherence over theoretical creativity; (3) major jurisdictions rely on aptitude testing combined with long-term supervised practical training. Building on these insights, the paper proposes an alternative model consisting of an aptitude-based selection process, short institutional training, and one to two years of supervised practical apprenticeship. The proposal aligns with Japan’s administrative culture and is fiscally feasible, supported by a detailed cost model. The study contributes a historically grounded critique of Japan’s selection system, a comparative analysis of competency-based training frameworks abroad, and a policy design tailored to Japan’s public-sector needs.