PURPOSE: To cancel the surrounding noise of a conference room by detecting a microphone used for transmission among plural microphones and applying subtraction processing between the output signal of the microphone and the output signals of the other microphones.

CONSTITUTION: A transmission sound signal is converted into a digital signal by an analog/digital converting circuit 46 and fetched by a control circuit 47. The control circuit 47 compares the quantity of each transmission voice signal level, specifies a microphone whose level is maximum as a transmission microphone, throws a changeover switch circuit to the position A and sends the signal to the non-inverting input of a differential amplifier 44. The surrounding noise collected by the other microphones is sent to the inverting input of the differential amplifier 44 via a resistance attenuator 45 while the changeover switch circuit is thrown to the position B. The component of the surrounding noise is cancelled and only the transmission sound signal is outputted.