A communication log aggregation device includes: a communicator that obtains flow information including one or more flow records and first statistical information for each flow from each of collection devices, the one or more flow records each including flow identification information included in a message received by at least one observer that is disposed in a control network system, the flow being classified based on the flow identification information, the collection devices each collecting the one or more flow records and the first statistical information for each flow from the message received by the observer; and a flow aggregator that generates aggregated flow information by performing at least one of the following: (i) selecting at least one of the one or more flow records, (ii) adding second statistical information, and (iii) deleting at least one of the one or more flow records, and outputs the aggregated flow information.