A relay device communicably connects an upper-level air conditioning system and a lower-level air conditioning system that use different protocols. An upper-side communicator is connected to a first bus, and transmits and receives communication packets to and from the upper-level air conditioning system. A lower-side communicator is connected to an outdoor device that is a master device, and transmits and receives communication commands to and from the lower-level air conditioning system. An association information memory stores association information obtained by associating an address in the upper-level air conditioning system allocated to each device in the lower-level air conditioning system with identification information on each device in the lower-level air conditioning system. A processor performs a mutual conversion on the communication packets and the communication commands based on the association information stored in the association information memory.