A cam 13 formed so that cam height and a cam operation angle continuously change, and constituted to be rotated integrally with a camshaft 11 and relatively movable in an axial direction thereof, a valve lifter 20 pressed by a cam surface of the cam 13 to advance and retreat a valve, and an accelerator shaft unit 40 for moving the cam 13 in the axial direction of the camshaft 11 are included. The accelerator shaft unit 40 includes an accelerator shaft 41 placed to be capable of reciprocating in the axial direction of the camshaft, and an accelerator fork 42 supported at the accelerator shaft 41 and engaging with the cam 13, and has an adjust mechanism 70 for finely adjusting a relative position in an axial direction of the accelerator fork 42 with respect to the accelerator shaft 41.