["Journal of Public Economic Theory, Volume 28, Issue 3, June 2026. ", "\nABSTRACT\nThis study clarifies the effects of population aging due to rising life expectancy on the ratio of public debt to GDP and economic growth rate using an endogenous growth model with social security. A previous empirical study demonstrates that population aging increases the ratio of public debt to GDP. Population aging can significantly impact national finances through the pay‐as‐you‐go pension system. Therefore, this study demonstrates that population aging increases the ratio of public debt to GDP by introducing a pay‐as‐you‐go pension system into the overlapping generations model. Additionally, we demonstrate that population aging due to rising life expectancies reduces the rate of economic growth under realistic conditions, which is consistent with the empirical results. Furthermore, although an increase in the replacement rate in the pay‐as‐you‐go pension system monotonically reduces the economic growth rate, there is a replacement rate that maximizes social welfare under certain conditions.\n"]