When an engine operates at low engine speed, the tip end of a cam nose of an intake cam is at a second position. As the engine speed increases and exceeds a first engine speed, the tip end of the cam nose of the intake cam moves to a first position. When the engine operates at high engine speed, the tip end of the cam nose of the intake cam is at the first position. As the engine speed decreases and becomes lower than a second engine speed smaller than the first engine speed, the tip end of the cam nose of the intake cam moves to the second position different from the first position.