A three-dimensional cam of a three-dimensional map state, having
uncountable cam profiles continuously ranged along a rotation axis
direction, and a valve lifter following a cam surface of the
three-dimensional cam are included, and a lift characteristic of a valve
is continuously controlled by relative motions of the three-dimensional
cam and the valve lifter. The three-dimensional cam has cam top portions
formed to have a smoothly ranged edge line substantially along the
rotation axis direction thereof, and a part of the edge line includes at
least one valley portion to take on a row of peaks. The cam peak portion
of the cam top portion which forms the valley portion of the edge line is
moderately set to be deviated toward a delayed side with reference to the
rotation direction of the cam from the other cam peak portions.