To impede easy opening operation from the outside so as to be safe.

An opened position fixing device provided with a stop plate 8 rotated by a handle 6 so as to protrude from and recede into an oblong hole 27 provided in a side wall 26 of a guide frame 5 is built in a rising part 2 of a lower frame of a sliding-out window. A sliding prate 15 reciprocated in the guide frame 5 in association with opening/closing of a sash door is stopped and fixed in a specified sash door half-opened state position by the tip of the stop plate 8 protruded from the oblong hole 27. After the slide plate 8 is retreated into the tip recessed position of the stop plate 8, the handle 6 is operated to recess the tip of the stop plate 8, and the sliding plate 15 is moved to fully open the sash door.