An electronic control unit includes: a first determination unit which determines whether a message received is an address claim message; a second determination unit which determines, when it is determined that the message received is an address claim message, whether a device name included in the address claim message received is a predetermined device name; and a third determination unit which performs a predefined determination process, when it is determined that the device name included in the address claim message received is the predetermined device name, and determine whether the address claim message received is an unauthorized message according to a result of the predefined determination process.