To detect only a pulse signal of a desired transmitting pulse interval from received signals including back scattered light pulse.

The waveform of a pulse received from the outside is shaped, and it is judged whether the pulse width of this waveform is within a prescribed pulse width or not. When it is within the pulse width, a gate pulse is outputted from a pulse width judging circuit 2. A delay circuit 15 outputs the gate pulse with a delay for a prescribed transmitting pulse interval. A switch 3 is laid in ON state over a prescribed period only when the gate pulse outputted from the delay circuit 15 is received to output the pulse train received from the outside to a code judging circuit 4.