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 as to a cause of the one or more condition events in a case where the one or more condition events have occurred, wherein the control device is configured to: (A) detect one or more condition events out of a plurality of condition events included in the plurality of rules; (B) specify a first conclusion associated with the one or more detected condition events based on one or more rules of the plurality of rules; (C) perform a detection possibility judgment as to whether the monitoring system can detect the condition event or not, for each of one or more condition events out of the plurality of condition events included in the plurality of rules; and (D) calculate a first index value indicating a certainty of the first conclusion being the cause, based on an existence of detection of one or more condition events associated with the first conclusion and a result of the detection possibility judgment.