PURPOSE: To easily execute the operation of communication using an acoustic coupler by providing a first means to detect a component interrupting the communication from data inputted from the acoustic coupler, and a second means to output a message corresponding to the detected component when it exceeds a value decided in advance.

CONSTITUTION: The inside of a main body 1 is equipped with a network control part 11, MODEM 12, communication control part 13, encoding/decoding part 14 and system control part 15 and between an acoustic coupler 2 and the network control part 11 of the main body 1, a noise detection filter 18 which is attachable and detachable through both connecting terminals is mounted When a first means 18 detects the component interrupting the communication from the data inputted from the acoustic coupler 2 and the detected component exceeds the value decided in advance, second means 4 and 15 output the message corresponding to the component. Thus, when executing communication, it can be easily judged whether surrounding noise interrupts the communication or not.