A valve system for the overhead valve type four-cycle engine disposing the intake and exhaust valves substantially symmetrically to each other with respect to the cylinder center line, wherein the valve stems are so tilted in the cylinder head that the valve bodies are in juxtaposition close to the cylinder center line. Guide cams are disposed around the circumferential surface of the crank shaft, respectively return back to the same starting point while making two turns, and respectively have one projection in the course of the turn, for driving the intake and exhaust valves. Between the guide cams and the respective intake and exhaust valve, a sliding piece, push rod and rocker arm are provided, and the projection on each of aforesaid cam is designed to be placed at such a position as to engage with the sliding piece to the opening timing of the intake and exhaust valves.