Abstract: An ATM exchange network system in which a first equipment and a second equipment are connected through a duplex communication line comprising a working line and a protection line. In this system, each of the first equipment and second equipment provides: a bridging unit which bridges the working line and the protection line on the transmission side; a switch for selecting the working line or the protection line on the reception side; a K-byte generating function for generating byte information containing information for realizing the APS; a function of receiving the K byte information from the protection line, analyzing the received information, placing the prescribed information in the K byte and realizing the APS; and a function of changing over the switch. Thus, the reliability with respect to a line failure in the ATM exchange network can be improved.