PURPOSE: To obtain an apparatus which is stable with a high response dispensing with any tachogenerator by arranging the output signal of a rotary encoder to be a speed signal varying in the code of the output signal with the direction of rotation.

CONSTITUTION: Outputs A0 and B0 of a 2-phase type rotary encoder which outputs a sine waveform fixed in the amplitude with the phase difference of 90° are inverted 3 and 4 separately and the inverted outputs and the outputs A0 and B0 are fed to an analog switch 9. The outputs A0 and B0 are added up 5 while the output B0 and the inverted output A0 are added up 6 and the addition outputs undergo a logic processing 10 after the shaping of the waveform. Based on the outputs of the logic processing, output signals positive or negative in the gradient depending on the direction of rotation of the encoder are fetched through the switch 9 and differentiated 11 to be fed to a filter 12. Thus, the output signal of the encoder provides a speed signal different in the signal code with the direction of rotation thereof.