Abstract: In an input/output circuit of a communication device, such as network control unit of a facsimile machine or transmission/reception circuit of a telephone set, an amplification factor of an amplifier circuit is changed without replacing the circuit components. The amplifier circuit includes operational amplifiers which amplify an input signal from a telephone line and/or an output signal sent to the telephone line. A central processing unit is provided in conjunction with the amplifiers, which outputs a control signal to the amplification circuit so that the amplification factor of the operation amplifier is changed responsive to the control signal.