To provide a means capable of easily holding a distance between a sash and a window opening part in a short time without using welding.

This fixing device comprises an engaging device 1 detachably fitted to the side outer frame of the sash 80 and an engaged device 2 fitted to the window opening 90 in which the sash 80 is installed.The engaging device 1 has a support piece 10 slidably fitted to the side outer frame of the sash 80, a movable part 20 having a slit hole 21 to allow the movable part to be moved forward and backward relative to the engaged device 2, a screw 30 inserted into the slit hole 21 and fixing the movable part 20 to the support piece 10 at any position relative to the engaged device 2, and a projected part 40 projectedly provided at the tip part of the movable part 20 vertically relative to the movable plane direction of the movable part 20 and engaged with the engaged device 2. The engaged device 2 has a hole 52 in which the projected part 40 of the engaging device 1 is fitted.