An electronic control unit is connected to an in-vehicle network bus in an in-vehicle network system. The electronic control unit includes a first control circuit and a second control circuit. The first control circuit is connected to the in-vehicle network bus via the second control circuit over wired communication and/or wireless communication. The first control circuit performs a first determination process on a frame to determine conformity of the frame with a first rule. The second control circuit performs a second determination process on the frame to determine conformity of the frame with a second rule, and, upon determining that the frame conforms to the second rule, transmits the frame to the in-vehicle network bus.