An air-conditioning system comprising: an outdoor unit functioning as a heat source device; and an indoor unit configured to condition air in an air-conditioned space, wherein: the outdoor unit includes an outdoor unit control unit configured to search for a relay unit configured to relay heat transfer between the outdoor unit and the indoor unit by exchanging heat of heat media, and an outdoor unit communication unit configured to communicate with an outside; and the outdoor unit control unit is configured to transmit a first communication command from the outdoor unit communication unit to a particular communication address, and when a communication response is returned to the outdoor unit communication unit, determine what exists at the particular communication address based on an identification ID transmitted from the particular communication address to the outdoor unit communication unit, the air conditioning system being characterized in that the outdoor unit control unit is further configured to set the relay unit as a control target when it is determined that the relay unit exists at the particular communication address, and set the indoor unit as the control target when it is determined that what exists at the particular communication address is not the relay unit or when the communication response is not returned to the outdoor unit communication unit. an outdoor unit functioning as a heat source device; and an indoor unit configured to condition air in an air-conditioned space, wherein: the outdoor unit includes an outdoor unit control unit configured to search for a relay unit configured to relay heat transfer between the outdoor unit and the indoor unit by exchanging heat of heat media, and an outdoor unit communication unit configured to communicate with an outside; and the outdoor unit control unit is configured to transmit a first communication command from the outdoor unit communication unit to a particular communication address, and when a communication response is returned to the outdoor unit communication unit, determine what exists at the particular communication address based on an identification ID transmitted from the particular communication address to the outdoor unit communication unit, the air conditioning system being characterized in that the outdoor unit control unit is further configured to set the relay unit as a control target when it is determined that the relay unit exists at the particular communication address, and set the indoor unit as the control target when it is determined that what exists at the particular communication address is not the relay unit or when the communication response is not returned to the outdoor unit communication unit. the outdoor unit includes an outdoor unit control unit configured to search for a relay unit configured to relay heat transfer between the outdoor unit and the indoor unit by exchanging heat of heat media, and an outdoor unit communication unit configured to communicate with an outside; and the outdoor unit control unit is configured to transmit a first communication command from the outdoor unit communication unit to a particular communication address, and when a communication response is returned to the outdoor unit communication unit, determine what exists at the particular communication address based on an identification ID transmitted from the particular communication address to the outdoor unit communication unit, the air conditioning system being characterized in that the outdoor unit control unit is further configured to set the relay unit as a control target when it is determined that the relay unit exists at the particular communication address, and set the indoor unit as the control target when it is determined that what exists at the particular communication address is not the relay unit or when the communication response is not returned to the outdoor unit communication unit.