PURPOSE: To improve engine performance, simplify function and construction, and dispense with high working accuracy.

CONSTITUTION: A valve system is provided with a rocker arm 1 for low speed and a rocker arm 2 for high speed, and a switch mechanism 4 having a piston 41 is mounted in the slide hole 11 of the rocker arm 1 for low speed. The rocker arm 2 for high speed is provided with a connecting end 24 which approaches the switch mechanism 4. The rocker arm 1 for low speed and the rocker arm 2 for high speed are driven by respective cams 18, 29 for low speed and high speed, but a valve 9 is opened/closed only by the rocker arm 1 for low speed at the time of low speed. On the other hand, at the time of high speed, high pressure oil is forcibly fed from the oil pressure port 31 of a rocker shaft 3 so as to push out the piston 41, which may be forced to abut on the connecting end 24 of the rocker arm 2 for high speed so as to open/close the valve 9 by the rocker arm 2 for high speed.