To provide an air conditioning system capable of easily additionally installing or replacing equipment.

An indoor unit has at least one of a first communication circuit (transmitting circuit 270, receiving circuit 290) for performing communications through two communication lines independent from power supply lines, and a second communication circuit (transmitting circuit 280, receiving circuit 300) for performing communications through one of the power supply lines and one communication line independent from the power supply lines, and the outdoor unit has a third communication circuit (transmitting circuit 110, receiving circuit 120) connected to the indoor unit having one of the first communication circuit and the second communication circuit, and performing communication with one of the first and second communication circuits, and a switch 160 for connecting one of communication terminals of the third communication circuit to one of the power supply lines when the third communication circuit is connected to the second communication circuit, and releasing the connection concerned when the third communication circuit is connected to the first communication circuit.