This survey measured the thermal environment of a bedroom in an apartment building during summer. The size of window opening was varied from wide to narrow, and windows are changed to opening south side or north and south side. The survey was performed at bedtime. The results show that indoor air temperature is closely related to the part of the building in which the room is located. Temperature showed a maximum difference of 1.5 K by room which part of apartment buildings is located. In addition, the results show that it is important to open two sides of a window rather than to operate mechanical air conditioner. It causes satisfactory without mechanical air conditioner, and save 194.4 Wh only during one night. In addition, the ventilation with opening window both sides is meritorious to take away body heat.