Background <p>Population aging, changes in employment patterns, and widening income disparities have caused concerns about equity in healthcare access within Japan’s universal health insurance system. This study examined income-related inequality and horizontal inequity in healthcare utilization in Japan.</p> Methods <p>Using repeated cross-sectional data from 827,168 individuals aged ≥ 20 years, collected from 13 waves of a nationwide population-based survey (1986–2022), trends in the concentration index and horizontal inequity in healthcare utilization were analyzed. A decomposition analysis of the concentration index was conducted to identify the key factors contributing to inequality in healthcare utilization.</p> Results <p>Horizontal inequity remained low over the study period after adjusting for healthcare needs; that is, actual healthcare use was closely aligned with healthcare needs. Although pretax income consistently contributed to an unequal distribution of healthcare utilization in favor of the rich, the impact was modest and was mitigated by the growing pro-poor contributions of residence-based health insurance plans and health insurance programs for older adults.</p> Conclusions <p>The results suggest that covering an increasing number of precarious workers with residence-based health insurance plans and providing financial support to the aged population through lower co-payment rates have provided protection against the increasing need for healthcare services caused by rapid changes in the demographic composition and labor markets in Japan.</p>