When a rotor performs an ordinary rotation operation, an axial position of the rotor is retained such that the rotor is rotatable with respect to a stator. When a parking brake switch is operated while a vehicle is at stoppage, a rotor slider slides the rotor in an axial direction against a preload of an axially preloading part. This brings an engaging portion of the rotor into engagement with an engaged portion of a housing. The reverse input holder retains the rotation angle of the rotor during the engagement due to the fact that the engagement between the engaging portion and the engaged portion is maintained against a reverse input load acting on the rotor as an external force that accompanies a reaction force of a pressing force of a brake friction member.