In this paper, an index of actual apartment sales prices for Korea is developed based on actual apartment sales price data from the Ministry of Land, Infrastructure and Transport and using the Case-Shiller method. While the existing actual apartment sales price index published by the Korea Appraisal Board  measures only the difference in two sales prices at different points in time, this new index also takes into account the time between the two transactions to enhance its accuracy. For instance, if no transactions have been made during a long period of time, this index takes into account the factors causing price fluctuations, such as depreciations. The new index seems to identify volatility better than the Korea Appraisal Board’s  index, which is calculated based on the same data. This makes the new index better suited for tracking short-term changes in apartment sales prices. Meanwhile, using the price diffusion model of Holley et al. (2011), an analysis is made of how apartment sales prices in the southeast areas of Seoul affect those in the Seoul metropolitan area overall. The result shows that apartment sales prices in the Seoul metropolitan area are heavily affected in the short term by prices in southeast Seoul and the surrounding area, but no clear long-term relationship was found.