To surely support a cylinder and a plunger even though a pulley shaft such as a hollow shaft is hardly provided with a sufficient thick-wall, and require just ordinary components in order to form a mounting boss for installing the cylinder and plunger on the pulley shaft, and realize a simple structure unnecessary for specially designed thick-wall.

The plunger 49 forming a secondary oil pressure chamber 26 of a secondary pulley 19 has a circumferential locking groove 18a on the outer periphery of the secondary shaft 18. A split ring-shaped cotter 50 is arranged on the locking groove 18a. A cotter retaining portion 49a housing the outside of the cotter 50 on a contact portion with the cotter 50 is formed on a mounting base of the plunger 49 to the secondary shaft 18. On an anti-engine side, the plunger 49 is fixed between the cotter 50 and a lock nut 51 by tightening the lock nut 51 from an engine side while the cotter 50 is housed in the cotter retaining portion 49a.