PURPOSE: To enable the detection of a high rotary speed, bt constituting the titled apparatus so as to direct the frequency deviation corresponding to a speed to one direction regardless of a rotary direction by changing over an exciting signal phase through a polarity discriminating circuit for discriminating the rotary direction.

CONSTITUTION: Detected voltage with frequency (f) satisfying the condition of the formula I from a resolver 1 integrally revolved with a rotor is treated with a wave form shaping circuit 4, a frequency-voltage converter circuit 5, a switch circuit 6 and a differential amplifier 7 and the output polarity thereof is detected by a polrity discriminating circuit 8. In this case, when polarity is judged as negative polarity, a switch 3 is changed over along with the switch 6 and the exciting voltage of the resolver 1 is changed over from Cosωf and Sinωf to -Cosωt and -Sinωt. By this method, frequency deviation Δf proportional to a rotation number is always directed to a definite one direction regardless of +Δf and a rotary direction and a high rotary speed can be accurately detected.