A vane rotor is rotatably placed in an inside of a housing. A lock pin, which is placed in an inside of a receiving hole of the vane rotor, is insertable into a fitting hole formed in a rear plate. A second pressure chamber formed in the fitting hole is communicated with an advancing chamber. An air discharge hole communicates between the second pressure chamber and the atmosphere in a state where the lock pin is inserted into the fitting hole. The air discharge hole blocks communication between the second pressure chamber and the atmosphere in a state where the lock pin is removed from the fitting hole.