An anomaly detection electronic controller performs anomaly detection processing and is connected to a bus, which a plurality of electronic controllers use for communication to communicate following a Controller Area Network (CAN) protocol. The anomaly detection electronic controller includes an anomaly detection processor that performs anomaly detection processing regarding a data frame. The anomaly detection controller also includes an anomaly detection processing requester that decides an anomaly detection processing timing in accordance with a state of a vehicle in which the bus is installed when receiving the data frame, the anomaly detection processing timing being a reception timing of one or multiple fields in the data frame. The anomaly detection processor further performs the anomaly detection processing regarding the data frame at the anomaly detection processing timing decided by the anomaly detection processing requester.