A gateway device is disclosed which processes only really necessary information for gatewaying and thereby prevents unnecessary increases in communication traffic on a communication bus to which data is sent through the gatewaying. When it is judged that the contents of received data concern a command that does not need gatewaying, the processing for gatewaying is inhibited. When it is judged that the contents of the received data show information that should be processed for gatewaying only when there is a change in the contents, the contents are compared with the latest contents of the same kind of information stored in a memory, to determine whether there is a change in the contents, and the processing for gatewaying is inhibited when it is determined that there is no change.