Objectives This study aimed to examine whether ministry-level parliamentary workload was associated with outpatient medical costs among Japanese national civil servants, who are exempt from the Labor Standards Act’s statutory limits on working hours.

Methods We conducted an ecological study of headquarters branches (“Kasumigaseki”) of ten major cabinet-level ministries, 2016–2023. Participants were Japanese national civil servants aged 18–50 years. The analysis included 6454 outpatient claims adjudicated in April of each year. The outcome was sex- and age-standardized per-capita monthly outpatient costs, estimated using Horvitz–Thompson weighting. Inter-ministry differences were decomposed by ICD-10 category. Parliamentary (Diet) workload was annual deliberation time of standing Diet committees responsible for each ministry’s policy areas, divided by insured headquarters members. Weighted least-squares regression examined the association between prior-year deliberation time and standardized outpatient costs.

Results Standardized per-capita outpatient costs varied more than twofold across ministries, from $19.8/month at the Ministry of Foreign Affairs to $42.7/month at the Ministry of Health, Labour and Welfare (MHLW). At MHLW, mental and behavioral disorders accounted for the largest above-average cost component. MHLW also had the longest deliberation time. Adjusting for year fixed effects, a 1% increase in deliberation time per civil servant was associated with a 0.25% increase in standardized per-capita outpatient costs (95% CI 0.04–0.46), driven by claims per capita rather than cost per claim.

Conclusions Greater parliamentary workload was associated with higher outpatient medical costs among Japanese national civil servants. Enhanced occupational health monitoring and support in ministries with persistently high workloads may be worth considering.

Key terms administrative claim; ecological study; healthcare utilization; Japan; long working hour; outpatient medical cost; workload

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