A network relay device compliant with network plug-and-play protocols is presented. The relay device relays messages between a network and a device unit having N service devices that provide a service in response to a request from a client on the network. In one embodiment the relay device has a description creating module configured to create a device description which describes service devices included in the device unit connected to the relay device. If one or more service devices of the device unit are inoperative, the description creating module creates a device description that does not include a description portion of the inoperative service devices and forwards the created device description to the client. In another embodiment, the relay device has a response module configured to respond to a device search request sent from a client. The response module responds to a device search request sent from a client if at least one service device in the device unit is in operative condition, while the response module does not respond to a device search request if all of the service devices in the device unit are in inoperative condition.