To prevent a malfunction due to a wrong connection by stopping supply of a power source if the wrong connection of a cable is made.

A transmission side unit 10 comprises an oscillation part 14A which outputs a frequency fa when input terminals 11A and 11B are applied with +/- DC voltages, respectively, and an oscillation part 14B which outputs a frequency fb when applied with voltages of opposite polarity. These outputs are superposed on the DC voltage and outputted from output terminals 12A and 12B. A reception side unit 20 comprises a frequency detecting part 27A that operates when input terminals 21A and 21B are applied with +/- DC voltages, respectively, and a frequency detecting part 27B that operates when applied with the voltages of opposite polarity. Relay contacts 29a and 29b are closed when the frequency detecting parts 27A and 27B detect the frequencies fa and fb, respectively. Thus, if a cable 8 is connected in opposite direction, no power is supplied to a motor M.