invention to outline its characteristics with fig2 showing the system configuration in fig2 a multiplexer demultiplexer 100 on the west side and another multiplexer demultiplexer 200 on the east side are inter connected by a private line 3 and an isdn line network 4 to back it up in the multiplexed data format on the private line as illustrated in fig3 a plurality of time slots 1 through 24 are divided into four groups in this example and the sets into which the time slots are divided are called links here in this example links 1 and 2 are supposed to be the sets of time slots not to be backed up and links 3 and 4 those of time slots to be backed up therefore in transmitting data from the multiplexer demultiplexer 100 to the multiplexer demultiplexer 200 of the opposite station signals are multiplexed in the format of time slot arrangement shown in fig3 and the multiplexed signals are transmitted via the private line 3 which signals are to be backed up is determined according to the relative importance and urgency of each signal when the private line 3 is in trouble only the links 3 and 4 are selectively extracted in the multiplexer demultiplexer 100 and sent out to the multiplexer demultiplexer 200 of the opposite station via the back up isdn line network 4 this arrangement makes it possible to back up and transmit data signals of a quantity matching the transfer speed of the isdn line which is slower than that of the private line 3 for instance by dividing data signal of 1 5.