PURPOSE: To convert optional address data in a serial code string by providing a setting means which sets a desired conversion data address in the serial code string, a conversion data setting means, and a means which switches data to be converted and conversion data.

CONSTITUTION: Transmission data set to a data setting part 21 is written in a storage part 23 by a write/read setting part 26 and the control of a write/read control part 25, and this data is read out and is transmitted as a CDT in accordance with the frame of a specific address of the CDT. In case of switching of data, the address of a frame to be switched and information indicating switching are set to an address setting and data switching setting part 29. When address coincidence is detected by an address coincidence detecting circuit 30, data in this address is converted in a data switching part and is transmitted.