PURPOSE: To decrease the number of interruptions to stations where no data link is established and to improve the processing ability, by making a Baud rate for data link establishment and the Baud rate for actual data transfer to have a different value from each other.

CONSTITUTION: When a station 1 transmits data to a station 3, a packet for data link establishment is transmitted to the station 3 in the Baud rate of low speed. After the station 3 returns the response for confirmation to the station 1, the station 3 designates that the succeeding data transfer is executed in the Baud rate of high speed to a network controller 6. On the other hand, a network controller 5 of a station 2 independently of the data link establishment is in the Baud rate mode of low speed, then the data transfer packet executed in the Baud rate of the said high speed is not correctly received, allowing to avoid useless interrupting processing to the station 2.