This study estimates the income replacement ratios of three pensions. i.e., the national pension, the retirement pension, and the private pension, and compares them with one another. Several findings emerge from empirical results using a simulation method. First, the poorer the subscriber, the higher the income replacement ratio in the national pension. However, three estimates by the income classes in both the retirement pension and the private pension are equal. Second, summing up the income replacement ratios of three pensions, the company workers under the national pension scheme have the lowest estimate of 54% and the highest estimate of 135.7% on the basis of a twenty-year-long subscriber, and the lowest estimate of 110.8% and the highest estimate of 168.2% under a forty-year-long basis. Meanwhile, the individually insured persons in the national pension have the lowest estimate of 38.2% and the highest estimate of 118.5%, and the lowest estimates of 78.0% and the highest estimates of 135.4% under a twenty- and forty-year-long subscriber, respectively. These results imply the following things: first, the incentive systems for purchasing three pensions all together need to be implemented. Second, the sufficient income security during the old age with the public or the private pension schemes is more likely to lessen the government finance burden in the future.