1. A monitoring system configured to perform a cause analysis of an event which occurs in any of a plurality of node apparatuses, comprising: a storage device; and a control device coupled to the storage device, wherein the storage device is configured to store a plurality of rules indicating a correspondence relationship between one or more condition events regarding any of the plurality of node apparatuses, and a conclusion to be the cause of one or more condition events in the case where the condition events have occurred, wherein the control device is configured to perform the following: (A) detecting one or more condition events out of a plurality of condition events included in the plurality of rules; (B) specifying the first conclusion associated with the detected condition event based on the rule; (C) performing a detection possibility judgment on whether the monitoring system can detect the condition event or not, for each of one or more condition events out of a plurality of condition events included in the plurality of rules; and (D) calculating the first index value indicating the certainty of the first conclusion being the cause, based on the existence of detection of one or more condition events associated with the first conclusion and the result of the detection possibility judgment.