An anomaly detection server is provided. The anomaly detection server is a server for counteracting an anomalous frame transmitted on an on-board network of a single vehicle. The anomaly detection server acquires information about multiple frames received on one or multiple on-board networks of one or multiple vehicles, including the single vehicle. The anomaly detection server, acting as an assessment unit that, based on the information about the multiple frames and information about a frame received on the on-board network of the single vehicle after the acquisition of the information about the multiple frames, assesses an anomaly level of the frame received on the on-board network of the single vehicle.