PURPOSE: To make capacity small, and lighten its weight, and concurrently enhance its responsiveness.

CONSTITUTION: A pulley 10 is rotatably installed at one end of a cam shaft 1. The pulley 10 has hydraulic chambers 15 and 16 which are partitioned by a vane 3 projected out of the cam shaft 1. The control of the inner pressure of the hydraulic chambers 15 and 16 permits the vane 3 to be held at a neutral position (b), a delayed angle position (a) where the vane is rotated by θ/2 to the upstream side in the direction A of pulley rotation, and at a lead angle position (c) where the wane is rotated by θ/2 to the downstream side, and also permits the cam shaft 1 to be rotated while being interlocked with the vane 3.