The present invention involves initiating communication between an intercom master unit and an intercom slave unit that is installed in an environment where a ringing sound output from a speaker would reverberate, outputting a ringing sound at the location where the intercom master unit is installed, picking up the ringing sound with the microphone of the intercom master unit, outputting the ringing sound from the speaker of the intercom slave unit, picking up reverberating sound with the microphone of the intercom slave unit, outputting the ringing sound which has returned as reverberating sound from the speaker of the intercom master unit, measuring the circuit voltage on output from the speaker of the intercom master unit, and verifying if the output is no less than a threshold in order to verify the communication capability between the intercom master unit and intercom slave unit.