A relay device is connected to two networks of a plurality of layered networks and connected to a first communication device directly under a first lower layer network that is on a lower level of the two networks. The relay device includes a storage unit that stores connection information including model identification information associated with each of a second communication device and the first communication device and information on the number of connected second communication devices or on a memory area assigned to each of the first communication device and the second communication device, the second communication device being connected to a lower side through a second lower layer network that is situated lower than the first lower layer network, the connection information being arranged in a format capable of specifying a layer of a network to which each communication device is connected.