To enable a medical worker to easily distinguish a slave unit inputting a voice which is outputted from a speaker of a master, when a plurality of slave units are selected and listening is performed.

The slave units is selected by a selection part 23 of a channel selecting unit 20, the slave unit equipped with a slave unit microphone inputting a voice of a prescribed level or above is specified out of the selected ones and an indicating part 24 of the channel selecting unit 20 is made to blink. Thus the slave unit equipped with the slave unit microphone inputting the voice of the prescribed level or above is indicated, and therefore the slave unit inputting the voice of the prescribed level or above by the slave unit microphone can be narrowed down from the selected slave units. Accordingly, the medical worker can easily distinguish the slave unit inputting the voice outputted from the master unit speaker.