In an engine-brake assisting system comprising a hydraulic pressure producing unit, an exhaust valve driving unit and a hydraulic circuit member which are constructed as discrete elements, the hydraulic pressure producing unit produces a desired hydraulic pressure by rotating a camshaft disposed on a cylinder head to drive an engine-brake assisting cam. The generated hydraulic pressure is then supplied to an exhaust cam of the exhaust valve driving unit, which causes exhaust valves to open and close at a timing different from a valve opening and closing timing during a normal operation, thereby giving the engine a large brake force. With this arrangement, the reduction of overall size of the system and consequent reduction of overall height of an engine can be achieved.