To provide a communication control method of an air conditioning system easy in additional installation and the like of 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 and performing communication with one of the first and second communication circuits, and a switch 160 for setting a connection or disconnection state between one of communication terminals of the third communication circuit and one of the power source lines, and has a step for setting the switch to one of the connection and disconnection states, a step for transmitting a signal to the indoor unit in each state, and a step for determining the connection of the first or second communication circuit on the basis of presence or absence of response.