An electronic control unit is connected to a network 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 network via the second control circuit. The second control circuit performs a first determination process on a frame to determine conformity of the frame with a first rule. Upon determining that the frame conforms to the first rule, the second control circuit transmits the frame to the first control circuit. The first control circuit performs a second determination process on the frame to determine conformity of the frame with a second rule. The second rule is different from the first rule.