PURPOSE: To simplify the adjustment of switching timing of a valve by, in a structure for closing and opening suction valves, exhaust valves or the like via a cam follower upon the rotation of a cam, providing an end surface on the side of the cam of the cam follower with an inclined surface and rotating the follower around an axis.

CONSTITUTION: A cam mechanism includes a cam 10 having a lobe formation 11 which projects from a basic circular formation 12. A surface 14 of the cam 10 engages an end face 15 of a bucket follower 16. The bucket follower 16 is slidingly located perpendicular to an axis of rotation of the cam 10, in a bucket guide (not shown). The end face 15 of the bucket follower 16 is provided with a pair of flat surfaces 11 and 18 which are inclined to the axis of the bucket follower 16, and these surfaces 17 and 18 are smoothly interconnected by a diametrical arcuate surface 19 having a radius equal to the radius of a cam base circle 12. Then, the bucket follower 16 can be rotated by a rotating apparatus including a rack 21 meshing with engaging teeth 22 formed on the outer peripheral of the bucket follower 16 so as to change the switching timing of a valve.