PURPOSE: To preform smooth change-over with less variation of a valve lift curve and restrain sound of tappets by increasing the diameter of a base circle of a high speed cam so as to absorb plays caused by errors manufacture and assembly of rocker arms.

CONSTITUTION: A minimum value Emin of a play E between an engaging part 20 of a high speed rocker arm 4 and a hydraulic piston 14 is absorbed by increasing the diameter of a base circle 12a of a high speed cam 12 so as to eliminate the risk of interference between the hydraulic piston 14 and the engaging part 20 of the high speed arm 14, and accordingly, a large part of the play can be absorbed in a condition in which smooth change-over can be made while the difference between the maximum play Emax and the minimum play Emin can be absorbed by a ramp part so that the variation of an equivalent cam lift curve, as shown by the solid line in Fig. 1 (b), is less that of the case in which the difference Emax-Emin between the errors is not absorbed as shown by the broken line, thereby it is possible to carry out smooth change-over from a low speed range into a high speed range and to restrain sound of tappets.