PURPOSE: To monitor the abnormality without retarding message transmission by adding a data for abnormality detection for scan transmission sent from an SVC console and monitoring a fault or an abnormal operation generated in each terminal equipment of a LAN system.

CONSTITUTION: An SVC console 8 applies sequential scan transmission at a period of 18(ms) and sends a message. In this scan transmission, the data of 8k word can be sent by one transmission but only 6k word is used actually, the remaining 2k word is not in use. Thus, the abnormality monitoring message 15 to terminal equipments 2,3,4 is superimposed on the 2k word not in use to monitor whether or not the fault or the abnormal operation takes place in each terminal equipment at the period of 1(ms).