An IDS ECU includes: an anomalous frame detector that detects an anomalous frame; a connector communicator that transmits an anomaly-related request frame to a connector that is a transmitter of the anomalous frame, to request a response from the connector, and receives, from the connector, an anomaly-related response frame generated by the connector based on the anomaly-related request frame and indicating the transmitter; a network anomaly determiner that calculates, from the anomaly-related response frame, the number of anomalous connectors indicating the number of connectors that transmitted the anomaly-related response frame, and determines that an in-vehicle network system is: in a first anomalous state when the number is 0; and in a second anomalous state when the number is not 0; and a network anomaly handler that handles the first or second anomalous state determined by the network anomaly determiner.