To suppress the effect of noise of an ambient environment or the like and to ensure excellent speech quality by setting S/N radio of an audio-compression signal transmitted/received between a door phone slave unit and an intercom master unit to a large value.

A slave unit audio-compression circuit 104 for compressing an analog audio signal input into a slave unit microphone 101 of a door phone slave unit 1, and a master unit audio-compression circuit 204 for compressing an analog audio signal input into a master unit microphone 201 of an intercom master unit 2 respectively compress utilizing the diode voltage characteristics of one pair diodes connected in the reverse parallel. A master unit audio expansion circuit 205 of the intercom master unit for expanding audio compression signal compressed via the slave unit audio-compression circuit of the door phone slave unit and a slave unit audio expansion circuit 105 of the door phone slave unit for expanding the audio compression signal compressed via the master unit audio-compression circuit of the intercom master unit expand utilizing the diode voltage characteristics of one pair of diodes connected in the reverse parallel.