Objective: This study assessed the regional disparities and the associated factors in the implementation of cardiac rehabilitation in Japan.Materials and Methods: Regional disparities were investigated by comparing the number of cardiac rehabilitation units in each of 47 prefectures in Japan based on the National Database of Health Insurance Claims Open Data published by the Ministry of Health, Labour, and Welfare. The relationships between the numbers of inpatient and outpatient cardiac rehabilitation units and the numbers of registered instructors of cardiac rehabilitation, board-certified physiatrists, and board-certified cardiologists were examined.Results: The region with the highest and lowest numbers of inpatient units showed 11,620.5 and 1,650.2 population-adjusted cardiac rehabilitation units adjusted per 100,000 population, respectively, corresponding to a 7.0-fold difference. Meanwhile, 4,865.3 and 238.6 units were present in the regions with the highest and lowest numbers of outpatient units, respectively, corresponding to a 20.4-fold regional disparity. Our analysis showed that the population-adjusted number of inpatient cardiac rehabilitation units was significantly associated with the population-adjusted numbers of registered instructors of cardiac rehabilitation (r=0.647, P<0.001) and board-certified cardiologists (r=0.445, P=0.002) but only marginally associated with the population-adjusted number of board-certified physiatrists (r=0.329, P=0.024). Moreover, the population-adjusted number of outpatient cardiac rehabilitation units was significantly associated with the population-adjusted numbers of registered instructors of cardiac rehabilitation (r=0.406, P=0.005) and board-certified cardiologists (r=0.450, P=0.002) but not with the population-adjusted number of board-certified physiatrists (r=0.078, P=0.603).Conclusion: Large regional disparities were observed during the implementation of cardiac rehabilitation. Increased numbers of cardiac rehabilitation instructors and cardiac rehabilitation practices are expected to eliminate these regional differences in cardiac rehabilitation practices.