PURPOSE: To improve transmission efficiency by outputting a token frame of a preamble which is longer than other transmission frame, when switching a network system of a random access type CSMA/CD system to a dispersion control type token passing system.

CONSTITUTION: A transmission origin address prepared in advance from an address storage part 5 is read out, and this address is set to a destination address 303 of the top priority token 300. Also, a self-address of a storage part 5 is set to a transmission origin address 304 of the top priority token 300. In such a state, the first priority token 300 is transmitted, and whether the token 300 has been transmitted normally or not is checked, and in case that it has not been transmitted normally, the top priority token 300 is transmitted again. Also, in order to obtain preferentially a right of transmission, a preamble 301 which is longer than other transmission frame is provided. When this token 300 is transmitted normally, the own communicating mode flag is set to a token mode and the processing is executed.