PURPOSE: To simplify the structure of an oil pressure supplying circuit, by varying the setting load to a return spring in each connection-switching-means and providing a means which switches and supplies more than one stages of oil pressure to an oil pressure supplying passages, which is commonly connected to each connection-switching-means.
CONSTITUTION: When an engine is on operation at a, say, medium speed, the oil pressure of an oil pressure supplying source 46 in an oil pressure supplying means 45 is regulated by a regulator on one side to a comparatively low oil pressure and is supplied to each oil pressure chamber 29a or 29b. Then, in the first, connection-switching-means 21a, a piston 23a is moved to the first rocker arm (7) side. Thus, the first and second rocker arms 7 and 8 are connected to each other and are driven by a high-speed cam 5. On the other hand, in the second, connection-switching-means 21b, the movement of a piston 23b is prohibited by a return spring 25b. Thus, the second and third rocker arms 8 and 9 remain uncoupled. Accordingly, an inlet valve on one side opens or closes in correspondence to the movement of the high-speed cam, while the inlet valve on the other side opens or closes in correspondence to the movement of the second, low-speed cam.