To reduce cost by simplifying a structure in a safety stay for ventilating a window, by which the opening of the window can be controlled at the place of a ventilation by a restraint arm, opening-control working by the restraint arm can be released in the case of necessity and the window can be opened and operated directly from a closed state to a fully-open place.

The stay for ventilating the window has a guide frame 5 fixed onto an opening frame 1, an engaging frame 6 fixed onto the window 2, the restraint arm 7 being supported by the guide frame 5 and controlling the opening of the window 2 at the place of the ventilation and a changeover tool 8 changing over and operating the restraint arm 7. A slide pin 12 slid and guided to the guide frame 5 is installed at one end of the restraint arm 7 and an engaging pin 14 capable of being engaged or disengaged with or from the engaging frame 6 at the other end respectively. When the restraint arm 5 is slid and operated along a guide groove 10 by the changeover tool 8, the window can be changed over to the place of the ventilation, where the engaging pin 14 is penetrated and engaged to an engaging groove 18, and a fully-open place, where the engaging pin 14 is slipped out from the engaging groove 18 and an engagement is released. Since the restraint arm 7 is slid and operated and the state of the stay is changed over, the structure can be simplified.