A handle locking device for slidable sashes of a window with a frame consisting of a section member, provided with a faulty-operation preventing system adapted to stop the closing movement of the handle when the sash (12) is not in a closed position in its frame (11), comprises an actuating element (15) movable against the action of a spring (21) between a handle-locked position and a handle-released position with a movement perpendicular to the sliding movement of the sash (12) and projecting from the handle body (13b) within the section member thickness. Said actuating element (15) is provided with an active free end (14) received in the section member thickness and designed to interfere in the sliding direction of the sash, by a cam-slipping action, with a locating element (20) on the window frame when the sash (12) is brought to a closed position, to move the actuating element (15) to the released position.