PURPOSE: To increase the area of a fluid passage without increasing the body size of a housing while miniaturize and lighten a device by forming a recessed part in the circumferential direction on the outer periphery of a cam ring, to form a fluid passage toward the inner periphery of said housing.

CONSTITUTION: A recessed part 9a is provided on the outer periphery, in the circumferential direction, of a cam ring 9, to form a fluid passage A toward the inner periphery of a housing 1. As a result, the fluid passage A in between the recessed part 9a and the housing 1 can obtain a defined sectional area without increasing the body size in the radial direction of the housing 1. And, the recessed part 9a formed on the cam ring 9 can reduce the weight of the cam ring 9, while weight can also be reduced by miniaturizing the housing 1. The section of the recessed part 9a formed on the cam ring 9 can be formed into a U-shape to increase strength.