Abstract: A SOH (101) of a transmission frame of SDH (SONET) is terminated at a sender-side transmitter (3 or 4) and is added to remaining data at a receiver-side transmitter (4 or 3). At the sender-side transmitter (3 or 4), the remaining data (102, 103), including an administrative unit pointer (102) is converted into ATM cells and the ATM cells are sent out to an ATM network (2). After that, at the receiver-side transmitter (4 or 3), the remaining data is restored from the received ATM cells. As a result, operation, administration, and maintenance are independently performed over sections disposed at the input and the output sides of the ATM network (2). Further, processes (conversion to ATM cells, restoring SDH frames) are performed irrespective of the difference between the administrative pointer (102) and a payload (103), and the number of the signal paths (104) multiplexed in the payload (103).