To accurately transmit power, while reducing size.

A friction plate 72 is provided inside a friction plate moving space 71 between a side surface of a vane 41 and a first right side plate 30R, and the friction plate 72 is always energized toward the vane 41 by a plate spring 73. When the pressure inside a vane housing groove 35a exceeds the predetermined pressure to be determined by spring force of the plate spring 73, the friction plate 72 separates from the side surface of the vane 41 to leak the pressure oil inside the vane housing groove 35a outside the vane housing groove 35a.