The design of openings is important for sustainable living in the cooling climate. Dweller’s habit of opening windows was investigated in nine Japanese detached houses using a window monitor originated by the author. The monitor measures the open width of sliding window and record daily and annual changes. The performance of a house and the living style of its dweller were investigated using questions to the dwellers. The investigation gave the following results. Windows were closed not only at night but also in the day time even in mild seasons in most houses. The average open widths were very small in mild and hot seasons in most houses. The daily and annual changes of open widths are very various. The open widths of windows are influenced by many factors; the type of a room, the number of a floor, the living schedule of dwellers, thermal performances of a house and the equipment for heating or cooling and the weather conditions. Therefore these factors have to be reconsidered to design effective windows.