A log generation method for generating a log of communication on an in-vehicle network includes: performing a plurality of determination processes for determining, by using different methods, whether or not a message sent to the in-vehicle network is anomalous; generating a log in accordance with results of the plurality of determination processes; and transmitting the generated log. In the generating, information items to be included in the log are determined in accordance with a combination of the results of the plurality of determination processes so that the log does not include identical information items.