A network relay apparatus relays data accompanied by additional information among plural networks through input ports and output ports. In the apparatus, a deciding unit decides, from the output ports, an objective output port to which the data inputted via any one of the input ports is to be inputted, on the basis of the additional information of the data inputted. An output unit outputs the inputted data through the decided objective output port. A producing unit produces diagnostic data to which the additional information including information indicating the decided objective output port is added, and a providing unit provides the deciding unit with the produced diagnostic data. Further, a fault diagnostic unit is provided to determine whether a fault has occurred in the network relay apparatus on the basis of information concerning the diagnostic data and the decided objective output port.