In the case of the backward rotation of the crankshaft in the same direction as the direction of the load torque, the unlocking requires a higher unlocking torque. Here, an explanation is made regarding the case when the rotational angle θ of the crankshaft tracks the crankshaft rotational angle instruction value tθ shown in the figure. During the period from the unlocking control start time t1 to the end time t2, without setting the target motor torque Tm for controlling the rotational position as the motor torque, only the unlocking torque TLoff is set; when t2 is reached, as TLoff equals 0, only the target motor torque Tm for controlling the rotational position is set as the motor torque. When this measure is not used, due to the mutual interference between the Tm and the TLoff, as indicated by the bold dashed line, the unlocking is delayed. However, by setting the motor torque to equal Tm during the period of t1 to t2, it is possible to avoid the problem caused by the delay in the unlocking.