In this study, indoor thermal environment, resident behavior of operating cooling device, and thermal comfort vote are investigated at the living room of apartment during summertime in Seoul. Based on the results of the investigation, indoor air temperatures that residents turn air conditioners (especially for cooling load in this research) on and off were found out. The relationships between outdoor weather condition and the number of days using air conditioner, and whether operating patterns of the devices were also found out. The acceptable thermal comfort zone is figured out from these results, and this research is expected to contribute to the development of household air conditioner. The results can be summarized as the followings; Residents turned the air conditioner on at $29.76^{circ}C$ of indoor air temperature, and $28.89^{circ}C$ of $SET^ast$. And turned the air conditioner off at $27.31^{circ}C$ of indoor air temperature, and $23.70^{circ}C$ $SET^ast$. Therefore, acceptable thermal comfort zone could be lied between these temperatures. If comfortable indoor thermal environment can be obtained with various architectural passive cooling techniques based on the results, energy consumption of cooling devices will be reduced.