An information system has a plurality of network apparatuses, a plurality of node apparatuses and a computer that analyzes a cause of an event that occurred. A storage resource for the computer stores rules including conditions indicating events that occur in the network apparatuses and the node apparatuses, and conclusion. The computer generates an aggregate internal condition object for aggregating information on the occurrences of events of a plurality of condition objects corresponding to the conditions of the plurality of network apparatuses, and associates the aggregate internal condition object with the condition objects corresponding to the conditions of the network apparatuses, condition objects corresponding to the conditions of the node apparatuses, and conclusion objects corresponding to the conclusions.