<p>A modem apparatus of power line transfer communication using a power line as a transmission path. The modem apparatus comprises an amplifier for amplifying communication signals to provide a pair of output signals that are differential signals having a phase difference of 180 degrees therebetween; a signal transformer for providing the amplified communication signals into the power lines; and a balance circuit connected on the primary side of the signal transformer for enhancing the circuit balancing. The balance circuit is constituted by a variable element the element value of which can be changed, and there is provided a common mode detecting circuit that detects a common mode current flowing on the secondary side of the signal transformer and that changes the element value of the variable element of the balance circuit such that the detected common mode current becomes small.</p>
