PURPOSE: To improve the transmission efficiency in response to the traffic.

CONSTITUTION: A traffic detection means 13 detects a traffic transferred in a network. Then access system changeover means 43, 53,... select the data access system to be the multiplex access in which some of plural nodes are simultaneously accessed or the token passing system in which only a node having a token is accessed in response to the traffic detected by the traffic detection means. Moreover, topology changeover means 17, 18 select whether a topology of the network is of bus type or a ring type in response to the traffic detected by the traffic detection means 13.