This paper examines how population aging affects the output effect of a government spending shock by using a panel data of OECD countries. The government spending shock is identified as a forecast error of government spending, and its output effect is estimated by using the local projection method. We find that population aging affects the output effect of the government spending shock. While in non-aging economies, government spending shock increases output significantly in both short- and medium-terms, in aging economies, output responses are not statistically significant.