PURPOSE: To decrease the length of the transmission line between data terminal equipments and to decrease the transmission propagation delay time by installing data transmission lines like a star around a line concentrator and arranging data terminal equipments impartially to each transmission line.

CONSTITUTION: Data transmission lines 1 are installed like a star around a line concentrator 4 and data terminal equipments 3 are arranged impartially to each transmission line 1 to decrease the length of the transmission line between data terminal equipments 3. The propagation delay time equivalent to the length twice that of a shortest transmission line is enough at maximum. Thus, the propagation delay time being a delay of a carrier detection time for the prevention of collision in the CSMA system is decreased, and the time when the carrier signal is not detected is decreased and the possibility of data collision on a transmission line is precluded.