To cope with both the signal superimposing system/signal separation system of a responder by providing a function for stopping modulation in a 1st modulating part by a detection result if the responder coping with a 2nd carrier frequency responds is detected.

If a responder discriminating routine is started, the output of a 2nd carrier (fL) is forbidden, and then a modulated 1st carrier (fH) is supplied for a fixed time. Then, the modulation of the 1st carrier is paused. By this pause, unmodulated 1st carrier is kept to be lead through. If information is transmitted from the responder side via this unmodulated 1st carrier, a detecting output is generated from a signal detecting circuit 26, so that whether or not a signal is detected is determined. Then, if there is the output of signal detecting, the responder on the other side is recognized as a signal superimposing system responder. On the other hand, a signal is detected from a detecting circuit 35, and an output is determined, as a result, if an output is detected, the responder on the other side is recognized as a signal separation system responder.