The synchronous motor driving apparatus including position sensors provided in the synchronous motor, a current polarity detection circuit for detecting the polarities of the currents in the respective phase windings of the synchronous motor, an inverter driving the synchronous motor, a motor speed calculation unit calculating the rotational speed of the synchronous motor depending on the output signals from the position sensors, a speed control unit outputting a first voltage adjusting component (q-axis current command value Iq*) to cause the rotational speed of the synchronous motor to approach a speed command value and a phase control unit outputting a second voltage adjusting component (d-axis current command value Id*) to cause the phase differences between the phases of the position sensor signals and of the currents in the respective phase windings of the synchronous motor to become a predetermined value.