A forward phase of a forward intermediate rotor is adjusted relative to a crank shaft. A forward stopper mechanism prevents further advance of the forward phase by engaging the forward intermediate rotor at a forward most-advanced phase. A forward locking mechanism locks the forward phase when reaching the forward most-advanced phase at a start time of the engine. A forward biasing member biases the forward intermediate rotor in an advance direction. The backward phase of the intake cam shaft is adjusted relative to a backward intermediate rotor by receiving a cam torque that is biased on average in a retard direction. A backward stopper mechanism prevents further retard of the backward phase by engaging the intake cam shaft at a backward most-retarded phase. A backward locking mechanism locks the backward phase when reaching the backward most-retarded phase at the start time of the engine.