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