To permanently hold a rotating position of an output shaft when a torque diode inserted to an irreversible rotation transmission system is locked on by reduction of input torque to zero.

After completion t1 of control to make a crankshaft rotating angle θ become a target value θs by motor torque Tm, when the motor torque Tm is reduced toward zero, the motor torque Tm is not reduced to zero at once at t1 but is reduced at a predetermined time change gradient ΔTm to finally reach zero. The reduction speed ΔTm of the motor torque Tm is set equal to or less than an upper limit value ΔTm_max of motor torque reduction speed which enables transition of the torque diode from a lock-off state to a lock-on state caused by reduction in the motor torque Tm to be performed while the crankshaft rotating angle θ is maintained at the target value θs, and is preferably defined to satisfy ΔTm=ΔTm_max.