PURPOSE: To tense a belt with a suitable tension by providing a toggle mechanism for drawing a belt holding member and a driving member together to an operating position and holding them.

CONSTITUTION: Toothed driving belts 21 and 22 extend around the peripheral edge frame of a window to operate a fastening means on each position of the frame. The end part 21 of the belt is held by a belt holding member 28. A driving member 13 can move along a part of the frame of the window. Toggle mechanisms 24 to 31 are arranged so as to draw the belt holding member 28 and the driving member 13 together to an operating position and hold them at this position. Thus, the belt can be properly tensed with suitable tension.