Abstract: The balancing bridge circuit is provided between a four-wire circuit and a two-wire circuit, to provide a thin-type balancing bridge circuit having satisfactory transmission characteristics and a MODEM and an exchange and a repeater of a telephone circuit having this balancing bridge circuit built therein. The transmitting terminals of the four-wire circuit and the connection points of the balancing bridge circuit are connected together through the photo-coupler. The receiving terminals of the four-wire circuit and the connection points of the balancing bridge circuit are connected together through the photo-coupler. The terminals of the two-wire circuit are connected to the connection points of the balancing bridge circuit. By the presence of the compact photo-couplers, it is possible to ground the transmitting terminal and the receiving terminal together. Common-mode noise reaching the terminals is not transmitted to the receiving terminals.