PURPOSE: To make it possible to digitally detect a speed from a low rotary speed to a high rotary speed, by detecting phase difference by using a phase locked loop control circuit.

CONSTITUTION: A wave form shaping circuit 31 inputs a frequency modulation signal with frequency of fo±fθ induceed to the secondary winding 13 of a synchroscope 10 and performs the wave form shaping thereof to obtain a digital square wave. A phase comparator 33 outputs the digital square signal subjected to wave form shaping in the wave form shaping circuit 31 and inputs the output signal of a frequency divider 32 to output a predetermined pulse signal. That is, control is performed so as to coincide the phases of two pulse signals with frequencies fo±fθ, fo±θ' inputted to the phase comparator 33 and, at this time, the output of the phase comparator 33 comes to "L" while a deviation counter 34 stops counting and the count data thereof comes to a constant value fo. This data fo comes to a digital rotary speed as it is. Therefore, a speed from a low rotary speed to a high rotary speed can be digitally detected.