Even though a study of airtightness performance of apartment and detached house have been done constantly, there are few of studies of multi-family housing which increasing every year. Therefore, this study analyzed airtightness performance of 20 households of one room in Daejeon to investigate airtightness performance standard. All experiments were performed under the same conditions except sealing windows to investigate airtightness performance without sealing windows (natural condition) and airtightness performance with sealing windows of studio apartment. As results, (1) average ACH50 without sealing windows was 19.2/h for pressurization, and 12.8/h for depressurization and (2) average ACH50 with sealing windows was 16.0/h for pressurization, and 10.7/h for depressurization and ACH50 in both condition, ACH50 under pressurization was about 50% higher than that under depressurization. Throughout this experiment, we can figure out that about 16% of air infiltration rate is occurred in windows, and the other 84% is occurred in rest of places such as Junction structure, socket and ventilating opening.