Abstract: In a telecommunication system having an upper hierarchy device including duplicated transmit/receive circuits connected to lower hierarchy devices through time-division multiplex transmission lines, data to be transferred is divided into data blocks each having predetermined length. An information indicating validity of the transmitting data and an information specifying one of the duplicated transmit/receive circuits which transmits the data are given to fixedly allocated time slots as an identifier. The identifier and the data block having predetermined length are sent to an arbitrary time slot of a time-division multiplexing transmission line as one frame. In a receiving device, the data on the time-division multiplexing transmission line are taken in sequentially. Since it is possible to identify a sending transmit/receive circuit by monitoring only a specific time slot in which the identifier is located and controlling the system, an effective use of the transmission line is realized.