An air conditioning system including a first communication line, a second communication line, and at least one common power supply line coupling an indoor unit and an outdoor unit. The indoor unit including at least one of a first communication circuit and a second communication circuit, the first communication circuit configured to communicate through the first and second communication lines independent of the at least one power supply line, and the second communication circuit configured to communicate through the at least one power supply line and one of the first and the second communication lines. The outdoor unit including a third communication circuit with a plurality of communication terminals coupled to the indoor unit and configured to communicate with at least one of the first and second communication circuits, and a switch configured to connect one of the plurality of communication terminals of the third communication circuit to the at least one power supply line in a state where the third communication circuit is connected to the second communication circuit and configured to open a connection between the communication terminal of the third communication circuit and the at least one power supply line in a state where the third communication circuit is connected to the first communication circuit.