PURPOSE: To provide the communication system which constitutes a bus type dual loop communication system where a master station can successively access slave stations in the opposite direction.

CONSTITUTION: A master station 1 and plural slave stations 3A, 3B, 3C, and 3D are connected to a bus type dual loop structure by a transmission line, and the master station 1 successively accesses slave stations by a primary communication line 11 when the communication line is normal. Slave stations 3A, 3B, 3C, and 3D respond to the master station 1 by the accessed communication line 11; and if the abnormality of the communication line occurs, the master station successively accesses a prescribed slave station by the primary communication line 11. When the response from the prescribed slave station is not returned within a prescribed time after transmission of transmission data, the master station switches the communication line to a secondary communication line 12 and accesses the prescribed slave station from the other end station side of the loop communication line, and the master station 1 switches the communication line to the primary communication line regardless of the response from this station is returned within the prescribed time after transmission of transmission data or not. The master station repeats the access to the next station; and after removal of the faulty position, the master station performs communication only with the primary communication line 11 again.