PURPOSE: To prevent lowering of driving efficiency by detecting phase current flowing through a piezoelectric body thereby detecting the driving state of a motor.

CONSTITUTION: Driving frequency of a ultrasonic motor is determined by the phase difference between phase current flowing through stator electrodes 1-1, 1-2 and voltage to be applied on a piezoelectric body. Phase current is detected through a detecting resistor 12 while basic wave component of driving frequency is separated through a bridge T type active bandpass filter 2 and shaped into a sine wave. At this time, an analog switch 13 is arranged at the prestage of the filter 2 in order to cut only the rectangular part of the phase current. The detected current waveform is converted through a comparator 3 into logic level pulses and a phase comparator 4 compares the phase of the driving voltage and the current. Oscillation frequency of a voltage control oscillator 6 is determined according to the phase difference.