A monitoring system performs cause analysis of an event occurring in any of a plurality of monitoring-target objects to be monitored based on a rule. In this case, the monitoring system makes a detection during the analysis time width and determines a plurality of conclusions based on an event corresponding to the condition for determining the conclusion. Moreover, the monitoring system performs one or more of (A) displaying change of certainty to be used for determination of a conclusion in a case where the analysis time width is assumed to be changed, (B) performing sort display of the determined conclusion based on an index value showing an affected range and (C) calculating the analysis time width based on the index value.