A gateway that notifies a fraud detection server located outside a vehicle of information about an in-vehicle network system including an in-vehicle network includes: a priority determiner that determines a priority using at least one of: a state of the vehicle including the in-vehicle network system; an identifier of a message communicated on the in-vehicle network; and a result of fraud detection performed on the message; a frame transmitter-receiver that transmits and receives the message communicated on the in-vehicle network; a frame interpreter that extracts information about the in-vehicle network based on the message received by the frame transmitter-receiver; and a frame uploader that notifies the fraud detection server of notification information including the priority and the information about the in-vehicle network.