PURPOSE: To prevent influence owing to an external interference cause or disturbance and to prevent perception by means of the interception of a third person by setting the waveform of a unit signal constituting a code to be complicated, and transmitting and receiving a communication content by means of the judgement of a level in plural points of the waveform concerned.

CONSTITUTION: A signal forming the code after t1 passes after the generation of a start pulse is transmitted. The signal has difference positive and negative waveforms. The negative signal waveforms are N1 and N2. The positive signal waveform is N3. N1, N2 and N3 are the signals of one unit of the code. Normally, the plural signals of the same unit are continuously transmitted. A demodulation output signal from a receiver 30 is high frequency-amplified in a high frequency amplifier 31 and is added to a bypass circuit 32. A positive/negative discrimination circuit 33 discriminates the positive and negative signals among the demodulation signals. The start pulse and the signal N3 are added to a start pulse driving circuit 34 and the delay operation of a timer 38 is started. A threshold circuit 35 has plural thresholds, investigates whether the level of an input signal is more than the threshold or not. Then, a signal '1' is added to a forward transmission logical circuit 36 when it is more than the threshold, and a signal '0' when it is less than the threshold.