A stopper piston is fitted into a fitting hole when a vane rotor is located at an approximately intermediate position between a most retard position and a most advance position with respect to a shoe housing. When the stopper piston is fitted into the fitting hole, a relative rotation of the vane rotor with respect to the shoe housing is restrained. When the stopper piston rotates to an advance side with respect to the shoe housing over the intermediate position at which the stopper piston is fitted into the fitting hole, a damper chamber communicates with an advance oil pressure chamber through a through hole, an oil passage and a recess space. When the vane rotor rotates to a retard side including the intermediate position, the damper chamber is sealed hermetically, and therefore the moving speed of the stopper piston toward the fitting hole decreases.