A driving device controls a permanent-magnet synchronous motor having a permanent magnet in its rotor using a voltage-type inverter supplying drive power for the synchronous motor, makes the torque of the synchronous motor and the d-axis current flowing in the synchronous motor in the direction of the magnetic flux generated by the permanent magnet approach their own command values, and performs weakening field control by decreasing the d-axis current. To perform the above described control without complicated d-axis current command value operations or temperature amendments to motor constants, the driving device includes a proportional controller for outputting a d-axis signal proportional to the deviation between a d-axis current detection value and a d-axis current command value for the motor. A d-axis voltage command value and a q-axis voltage command value are determined according to the d-axis signal and a q-axis signal obtained based on the deviation between a q-axis current detection value, which is a component in the direction normal to the magnetic flux, and the torque command value for the motor. A voltage command vector is obtained from the d-axis voltage command value and the q-axis voltage command value. The voltage command vector is PWM-converted and a voltage-type inverter control signal is generated.