is rotated towards the forward direction moreover upon starting of the motor 13 no output is produced from the limiting amplifier 16 hence a transistor q4 is in an off state and a transistor q5 is in an on state however a l level signal is applied to a base of a transistor q6 through the terminal 10 and puts the transistor q6 in an off state upon constant rotation the rotational frequency signal obtained from the frequency generator 14 as the rotor of the motor 13 is rotated is supplied to the limiting amplifier 16 through an amplifier 15 a waveform shaping operation is performed with respect to the rotational frequency signal at the limiting amplifier 16 the output of the limiting amplifier 16 is supplied to the phase comparator 12 on one hand wherein the phase of the output from the limiting amplifier 16 is compared with the phase of the reference frequency signal obtained from the reference oscillator 11 an error output signal obtained as a result of the above comparison of phases is supplied to the motor driving circuit 17 accordingly the motor 13 is controlled by the motor driving circuit 17 so as to rotate at a constant speed according to the above reference frequency signal on the other hand the output of the limiting amplifier 16 is applied to a base of the.