To save a necessary transmission capacity by rewriting channel positional information in a multiplexed signal in a transmission line based on channel positional information on a private line.

A signal block BLK 1 housing respective 1 and A being the combination of an ID number and data is inputted to a line exchange part CS to read in the ID number and data A. Next, under the control of N ID rewriting part IDT, the line exchange part CS rewrites only the ID number 1 to 3 and sends the ID number 3 and data A as a signal block BLK 4. Similarly a signal block BLK 2 is read into a buffer memory part IDT 2 to send to a signal block BLK 5, and a signal block BLK 3 is read into the buffer memory part IDT 2 to change the ID number 3 to 1 and then to sent to a signal block BLK 6. Thereby the ID number 3 of the signal block BLK 1 is rewritten to 1 and the ID number 1 of the signal block BLK 3 is rewritten to 3 to exchange lines between channels 1 and 3.