A loop transmission system having a plurality of data processors (11-18) connected through respective transmission stations (21-24) with a common loop transmission line (3) is disclosed. This system has a concentrator (4) connected with a plurality of transmission stations (21-24) through respective loop transmission lines (3). The concentrator (4) changes the order of connection, said loop transmission lines (3) thereby to change the order of connection of said transmission stations (21-24) on said loop transmission lines.