An electronic control unit is connected to an in-vehicle network bus in an in-vehicle network system including a plurality of apparatuses that perform communication of frames via the bus. The electronic control unit includes a first control circuit and a second control circuit. The first control circuit is connected to the bus via the second control circuit over wired communication and/or wireless communication. The second control circuit performs a first determination process on a received frame received from the bus to determine the conformity with a first rule related to at least a reception interval, and, upon determining that the received frame conforms to the first rule, executes a predetermined process based on the content of the received frame. The first control circuit performs a second determination process on the received frame, received via the second control circuit, to determine the conformity with a second rule different from the first rule.