PURPOSE: To discriminate a fault position speedily and securely in case of the occurrence of a fault by deciding on fault contents from the correlation between physical monitor information and logical monitor information.

CONSTITUTION: If physical alarms 1', 3', 5', 6', and 7' and logical alarms 5", 6", and 7" are generated within a specific time, the alarm generation is stored in a current-state storage area 23 and compared with the contents of a last-state storage area 24 to rearrange them in the increasing order of influence, e.g. 1', 3', 5', 6', 7', 5", 6", and 7". Then, a table area 26 showing the relation between fault alarms and influence alarms is referred to on the basis of the alarm 1' with the 1st level to extract succeeding influence alarms 3', 5', 6', 5", and 6", and the influence alarms in a sorting area 25 are erased. Thus, when the contents of the sorting area 25 are all processed, the remaining alarms, e.g. 1' and 7' in the sorting area 25 are real fault alarms.