The purpose of this paper is to estimate effects of healthy city policies on reducing social costs. The analyses were dune at the cities, counties, and communities levels in 2009, and covered Seoul Metropolitan Area(SMA). For estimation of reducing social costs, it developed a system dynamics(SD) model that analyzed causal relationships between physical inactivity rates, the number of deaths, medical expenses, and total social costs. Simulation period of SD was from 2009 to 2030. Three alternatives were proposed with combinations of length of bike lanes, number of bus routes, crime rates, self-reported good health status rates, and obesity rates. The total estimated cost of physical inactivities from 2009 to 2030 was 31.9 trillion won from the future forecast without policies. As a result of simulations with three alternatives, there were economic benefit approximately from 119.7 billion won to 1.16 trillion won. This study contributed to better understanding the economic benefits of healthy cities that were associated with design of built environment and physical activity. It also emphasized the importance of healthy cities planning as one of national welfare polices.