To specify a channel position by a channel for data transfer by detecting a special character K28.5 in transmission signals received by the channel 1, replacing it with K28.1 or K28.7 multiplexing them with the transmission signals from the input channels of the channels 2-(n), sending them out to a multiplex transmission line and performing a reception processing.

The reception part 612-1 of a multiplexer 610 specifies the bit position and word boundary of the code of the 1st-3rd characters of the channel 1, converts them to the signals of 10 bits and transfers them. A transferred code detection and conversion part 613 detects that the 1st character is K28.5, replaces it with K28.7110 and transfers it to a multiplex transmission part 614 with the 2nd-3rd characters. The reception parts 612-2-612-n specify the bit position and word boundary of the code of the 1st-3rd characters of the channels 2-(n), convert them to the signals of 10 bits and transfer them to the multiplex transmission part 614. The characters of the channel 1 and the channels 2-(n) are multiplexed and transmitted to the multiplex transmission line 601. On a reception side, synchronization establishing time is shortened.