the terminal s of the segment electrode 502 for detecting the vibration of the surface wave driven motor at that time is v in fig5 the characteristic of the number of revolutions of the motor and the characteristic of the detected voltage for the input frequency under certain extraneous conditions are as indicated by n 1 and v 1 respectively and both of them become maximum at an optimum input frequency f 1 also when the extraneous conditions have changed the characteristic of the number of revolutions of the motor and the characteristic of the detected voltage for the input frequency are as indicated by n 2 and v 2 respectively and at this time n 2 and v 2 are maximum for the optimum input frequency f 2 in this manner it has been empirically found that under certain extraneous conditions the frequency which gives a peak to the number of revolutions n of the motor and the frequency which gives a peak to the detected voltage v are substantially coincident with each other if in spite of a change in the extraneous conditions the input frequency is still maintained for example at f 1 both the number of revolutions n of the motor and the detected voltage v will be greatly reduced so the rotation control device of fig4 is designed such that the oscillation controller 108 controls the oscillator 101 in accordance with the voltage detected from the terminal s of the.