PURPOSE: To make an automatic linking possible by communicating periodically with a delinked station in a data link system where n-number of slave stations correspond to one master station.

CONSTITUTION: Communication data for confirming a linking is transferred from a transmission data storage area 3 to a transmission data storage buffer 4 in the master station (station M) side and is transmitted to a slave station (station T). The station T stores received data in a receiving data sotrage memory 9 and takes out a response to this data from a transmission data storage memory 10. The station M stores data from the station T in a receiving data storage buffer 6 and transfers it to a receiving data storage memory 5. At this time, a command is transferred to a command buffer 8 to confirm whether the communication is performed normally or not; and if it is abnormal, the number of the communication is stored in an abnormal station storage buffer, and this number is tranferred to the abnormal station storage buffer after the communication to all slave stations.