PURPOSE: To close a window tightly by connecting a motor to a drive output member, and detecting the torque applied to the drive output member with a means of a drive connection part, and stopping the motor with a means to be operated by the detecting means when the torque achieves the predetermined value at the time of closing the window.

CONSTITUTION: When closing a window, in order to obtain the rotation of a drive output member 40 in the reverse direction, a motor 20 is operated so as to drive in the opposite direction, and a chain 96 is moved into a foundation of a housing. When the torque of a chain driving sprocket 95 and the drive output member 40 achieves the predetermined value, the motor 20 is stopped. When a window sash reaches a closing position, the torque is increased immediately by the rotation of the motor 20 for continuously driving the drive output member 40 in the window closing direction.