Objective: The Japanese government’s physician workforce reform, which commenced in April 2024, introduced regulations on physicians’ working hours. However, in areas facing physician shortages such as rural regions and surgical medical specialties, healthcare provision relies heavily on the extended working hours of each physician. The anticipated impact of this reform, when implemented, was significant.Materials and Methods: Using publicly available government data, we estimated the current working hours of physicians in various medical specialties in each prefecture across Japan. Subsequently, we calculated the ratio of surplus or deficit physicians when hypothetically assuming that all physicians adhered to the regulatory upper limit of 58.4 working hours per week nationwide.Results: Assuming that all physicians work to the regulated maximum, there would be a shortage of doctors in various medical specialties across Japan, such as surgery, neurosurgery, orthopedic surgery, obstetrics and gynecology, and emergency medicine. Geographically, shortages of doctors are observed in rural prefectures such as those in the Tohoku region, particularly in emergency- and surgery-related specialties, indicating a critical shortage of physicians in rural areas. Additionally, it has become evident that even in medical specialties with a calculated surplus of physicians nationwide, the margin of surplus is generally only a few percentage points.Conclusion: Currently, rural areas and surgical medical specialties in Japan have limited leeway in the physician workforce, and the strict application of workforce reform may lead to a severe shortage of physicians in these areas. It is noteworthy that as similar reforms may subsequently be implemented in other countries, analogous challenges would arise. Thus, the implementation of workforce reform requires a flexible approach to minimize its negative effects, which widen the existing disparity in the workforce.