PURPOSE: To allow operation of a synchronous motor as the prime mover for an electric automobile by regulating the phase difference angle and the frequency of the synchronous motor.

CONSTITUTION: A current accelerator stepping amount θ is inputted to set a basic phase difference angle δ0 (step 1, 2) and a phase difference angle correction amount Δδ is set depending on the increase/decrease of θ thus setting a phase difference angle 3 (step 4-11). Phase control is then conducted (step 12) and the rotational frequency fR of a rotor 22 is detected (step 13). Subsequently, the inverter frequency fI is matched with the rotational frequency fR so that the rotational speed of the rotor 22 matches the microfluctuating rotational speed of output shaft 23.