This invention is provided with: an adapter ring (cam ring housing member) for defining, between the adapter ring and the outer circumference of a cam ring, a first fluid pressure chamber and a second fluid pressure chamber for causing the cam ring to move with respect to the rotor by the pressure difference therebetween; a seal housing groove formed on the inner circumference of the adapter ring; and a slipper seal interposed in the seal housing groove, the outer circumference of the cam ring sliding against the slipper seal, and the slipper seal partitioning the first fluid pressure chamber and the second fluid pressure chamber when the cam ring is moved. The slipper seal is formed as a thin-plate shape having a square cross-section.