A rear plate is provided which encloses an axial end opening of a housing body. A lock pin is configured to travel forward and backward in a slide hole formed in a first vane of a vane member, and an annular lock-hole-forming part is press-fitted to a retaining hole in an inner end surface of the rear plate, forming a lock hole in the rear plate. A projection having a flat distal end surface is formed at one circumferential end side of an inner peripheral surface of the lock-hole-forming part. Recesses that are continuous with the inner peripheral surface of the lock hole are formed on corresponding sides of the projection in the circumferential direction of the lock hole. This allows smooth engagement of the locking pin in the lock hole, and permits smooth supply and drainage of hydraulic pressure to and from the lock hole.