A conventional solid protection device disposed on an exposed side of a cylinder for actuating a working device had a possibility of being permanently deformed by contacting an obstacle so as to be prevented from sliding properly corresponding to the telescopic movement of the hydraulic cylinder. An object of the invention is to provide a hardly deformable protection device. To protect an exposed side of a cylinder for a working device, a protection device includes a guard member and a slide guide into which guard member is telescopically fitted. Slide guide is attached to a cylinder tube, and a tip of guard member is attached to a tip of piston rod. Guard member is a flat plate made of spring steel. A width of guard member is larger than a diameter of the piston rod. Notches are provided on lateral opposite sides of a basic side end of guard member. Sliders are fitted to respective notches. The top of guard member is bent into a roll-shape so as to serve as a pivotal portion.