Investigations on the thermal environment, residents' cooling behavior, and residents' thermal comfort vote in apartments' living rooms in Seoul during summer were conducted. The relationship between outdoor weather condition and the number of days using air conditioner during whole summer was found out. Temperatures that residents turn air conditioners on and off were also found out as 29.76°(28.89 SET*) and 27.31°(23.70 SET*) of indoor air temperature. Therefore, acceptable thermal comfort zone at personal house could be laid between these temperatures, and it is higher and wider than chamber experiment results. If indoor thermal environment can be kept in this figure by various passive techniques of architecture, the energy consumption resulting from using cooling devices would be reduced.