A window stop device is adapted for limiting sliding movement of a window sash along a rail, and includes a restraining unit and a lever unit pivoting on the restraining unit between a locked position and an unlocked position. The restraining unit includes a pair of clamping members for flanking the rail. In the locked position, pressing arms of the lever unit press the clamping members, respectively, so that the clamping members clamp tightly the rail therebetween. In the unlocked position, the pressing arms are away from the clamping members so as to release the clamping members and permit sliding movement of the restraining unit along the rail.