The design of openings is important for sustainable living in the cooling climate. People’s habit of opening windows was investigated in Japanese houses using an original window monitor. Windows were not opened enough even in mild seasons and hot seasons, for the purpose of preventing crime or noise and securing privacy. The open width of windows depends on not only the indoor and outside conditions but also the living style of dwellers. This study shows that the open width of windows is influenced by the type of a room, the number of a floor, the living schedule of dwellers and the weather conditions. And the thermal performances of a house, equipments for heating or cooling and the living style of dwellers influence upon the annual change of open width of windows.