To provide a valve system able to suitably control valve opening timing and a valve opening period of a valve according to the operating state of an engine and having a simple structure.

In a valve gear of an engine, in which a locker 2 is interposed between a cam 5 provided on a cam shaft 3 to be rotated and driven in synchronism with a crank shaft and valves 8, an assist shaft 9 to be interlocked with the cam shaft 3 through a phase difference generating means is provided. An assist cam 11 for operating the locker 2 in the same direction as the operating direction by the cam 5 is provided on the assist shaft 9, and a controller for controlling a phase difference generating means in response to the operating state of the engine is provided. Therefore, valve opening timing and a valve opening period of the valve can be suitably controlled according to the operating state of the engine.