PURPOSE: To regularize an output torque characteristic in keeping with a variation in engine speed ranging from a low speed range to the high speed one.

CONSTITUTION: Low speed cams 47a, 48a, 57a, 58a and high speed cams 47b, 48b, 57b, 58b are all adopted in a cam opening or closing a valve in response to each of cylinders 1-6, thereby making a valve lift value alterable. In addition, an oil feed pipe 9 is connected to the cylinders 1-3 at the lower side via a solenoid valve 11, and the oil feed pipe 9 is connected to other cylinders 4-6 at the upper side via a solenoid 12 in the same manner. With these solenoid valves 11, 12 operated separately, the cam is changed over from the low speed cams 47a, 48a, 57a, 58a to the high speed cams 47b, 48b, 57b, 58b successively, thus the valve lift value is altered.