To provide a falling prevention mechanism of a double hung window capable of preventing a movable window from falling down abruptly irrespective of the loss of tension of a wire, thus improving the safety of the double hung window.

This falling prevention mechanism 100 of the double hung window is provided with the movable window 15 to move it up and down and freely along a groove of a vertical window frame 27, a main roller provided in the vertical window frame 27 and coming into contact with the movable window 15 to turn, an extending and contracting rod having a basic end part supported turnably on the vertical window frame 27 at the center of turning in the same direction as the direction of a turning shaft of the main roller and letting a brake roller supported on an extending and contracting tip part by the turning shaft in the same direction as the center of turning roll on the main roller, and a swing regulation means for regulating swing of the extending and contracting rod so that the brake roller is on a window contact side or the opposite side across a virtual line passing through the center of turning and the turning shaft of the main roller. Preferably, an energizing means for pressing the brake roller against the main roller is provided between the center of turning for supporting the basic end part of the extending and contracting rod and the brake roller.