Korea has experienced unprecedently rapid increase in life expectancy. More importantly the size of the unexpected increase in longevity is much bigger than that of any other advanced economies. This means the Korean government faces a huge financial risk. In this paper, we identify the cause of the overestimation in mortality to develop a new mortality forecasting model. Based on the population projection through the new mortality rate forecasts, we quantitatively measure the size of the longevity risk for the government. As a result of large longevity exposures of the Korean government, if realized, the longevity risk alone could push up debt-to-GDP ratios by 33.8 percent points as of 2060. We also make some suggestions on mitigating the longevity risk.