AIM: This study aimed to investigate the impact of the National Health Program in Japan ("Specific Health Check-ups and Specific Health Guidance") on diabetes prevention among working-age men with prediabetes. METHODS: This study used a regression discontinuity design, based on the program's criterion that the program starts at age 40 or older and that the intervention is provided only to prediabetic individuals with abdominal obesity, to assess the impact of the program on the diabetes incidence in a total of 49,848 men with prediabetes, aged 37-42 years. RESULTS: The National Health Program in which interventions were provided for individuals aged 40 years or over with both prediabetes and abdominal obesity was associated with a decrease in diabetes incidence rate equivalent to 10.1 reduction/1000 person-years. The relative risk was 0.75. However, among those without abdominal obesity and not subjected to the intervention, there was no significant change in the diabetes incidence at age 40. CONCLUSIONS: The National Health Program in Japan was associated with a decrease in the incidence of diabetes among working-age men with prediabetes and abdominal obesity and may have a meaningful impact among working-age men.