In an anomaly determination method for determining an anomaly in a received message, a plurality of messages which include messages that are periodic and each of which includes a first field having a fixed value and a second field having a variable value are each received as the received message, and one of a plurality of combinations to be used for determination each of which includes at least one of a plurality of anomaly determinations including an anomaly determination utilizing a reception timing based on the periodicity or the number of received messages, an anomaly determination utilizing the first field, and an anomaly determination utilizing the second field, is selected according to one or more criteria among available execution time of the anomaly determination method, a load amount, a data amount, and the number of messages.