PURPOSE: To prevent receiving data from being lacked and to simplify the function of a terminal device, by transmitting/receiving data only when opposite telephone sets are in operating state in a communication system acoustically coupling transmitters/receivers of telephone sets to each other to transmit/receive data.

CONSTITUTION: The titled acoustic coupling device is provided with a microphone M acoustically coupled with the receiver of a telephone set, an FLSI including an amplifier and a filter, an MLSI executing modulation/demodulation, etc. When a transmission request signal RS from a terminal device is erased during modulation/demodulation, the coupling device stops the transmission of a transmitting carrier independently of the detection of the receiving carrier by a detecting signal CD. When the detectig signal CD detects the disconnection of the receiving carrier, the coupled device stops the transmission of the receiving data RD to a terminal unit. When detecting the disconnection of the receiving carrier during modulation/demodulation, the coupled device stops the transmission of a transmissible signal CS and receiving data RD and also stops the transmission of the transmitting carrier.