A gateway apparatus performs frame forwarding between an external device connected to a first bus and controllers connected to a second bus. Each controller outputs a response frame upon reception of a communication frame from the external device. The response frame has an address field in which a private address is set. The private address is uniquely assigned to each controller in a private network space defined by the second bus. When the communication frame is directed to each controller, the communication frame is forwarded to each controller in sequence at different times to prevent collision between the response frames in the second bus. The private address set in the address field of the response frame is converted into a predetermined transmitting address specified in the first bus.