PURPOSE: To stably transfer data with less noise by superposing a pulse voltage indicating the rotation of a servo motor outputted from a position detecting circuit upon a power line after converting the voltage into a position signal formed of an alternating current and extracting the position signal superposed upon the power line by means of an extracting means incorporated in a controller.

CONSTITUTION: A control signal is superposed upon the power line (RL) 31 of an insulating transformer(TR) 15. On an encoder 20 side, a TR 24 extracts the control signal from the RL 31 and inputs the signal to a receiver IC4. The receiver IC4 transfers the control signal to a position detecting circuit 21 after converting the signal into a voltage fluctuating signal. The circuit 21 outputs a pulsative voltage indicating the rotation of a servo motor in accordance with the control signal. The pulse voltage is converted into a position signal formed of a high-frequency alternating current by means of a driver IC3 and the position signal is superposed upon the RL 31 by means of a TR 24b. On a controller 10 side, a TR 15 extracts the position signal from the RL 31 and a receiver IC1 converts the position signal into the voltage fluctuating signal and inputs the signal to a control circuit 11.