RELAY DEVICE, UNAUTHORIZED FRAME DETECTION METHOD, AND IN-VEHICLE DEVICE

A relay device is a relay device that, in an in-vehicle network to which a plurality of in-vehicle devices are connected, relays frames among the plurality of in-vehicle devices. The relay device includes: a reception unit configured to, in a case where an unauthorized frame is detected by an in-vehicle device among the plurality of in-vehicle devices, receives a request to start detecting an unauthorized frame from the in-vehicle device; and a detection unit capable of executing detection processing that detects an unauthorized frame among frames transmitted over the in-vehicle network. In a case where the request to start is received by the reception unit in a stopped state in which the detection processing is stopped, the detection unit transitions from the stopped state to an executing state in which the detection processing is executed.