PURPOSE: To transmit data through any of transmission lines by performing the management which of duplicated transmission lines is used by a main station at each to use a normal driver/receiver when any normal driver/receiver still remains in the station.

CONSTITUTION: The main station MST2 inquires about the state of the driver/ receiver 8A, 8B of each RST to all remote stations RST4 through transmission lines 3A, 3B of both A and B channels in a prescribed period and forms a managing table as shown in Figure in a memory. CHA=1 is obtained in this table when the 8A is normal, CHB=1 is obtained when the 8B is normal and any of or both of the 8A, 8B is normal, then BHAV=1 is obtained. Further, a channel used with priority is indicated on the PRIOR. Thus, in transmitting data to an RST, the managing table corresponding to the RST is reference and a channel designated in the PRIOR is used when the CHAV is at "1". When the operation is in error in this case, the other CH not indicated by the PRIOR is checked and when normal, this is used and retried.