PURPOSE: To send desired information in addition to an AIS signal by adding a data to a sent data while giving violation to the AIS signal at a fault of a reception between host groups and sending a desired data to a subordinate group through the use thereof.

CONSTITUTION: A control section 1 sends a transmission signal from a host group to a subordinate group with a desired transmission data 102 added thereto through provision of violation (code rule violation) to an AIS signal at the transmission of the AIS signal being an all mark signal when a reception fault takes place in the host group. When the AIS signal exists in the sent signal, a receiver side of the subordinate group detects the violation from the sent signal at an AIS detection circuit, recover the added data according to the violation at a recovery circuit 9 to obtain a reception data. Even when a transmission line of a secondary group being the host group is broken, a data is able to be sent through a transmission line of a primary group being a subordinate group, desired information is also sent in addition to the AIS signal and effective utilization is attained.