To provide an air conditioning system free from failure even when faulty wiring occurs.

This outdoor unit has a first communication circuit (transmitting circuit 110, receiving circuit 120) performing communication by using one of power supply lines and one signal line independent from the power supply line as communication lines, and the indoor unit has a pair of terminals (terminal SG, terminal S1) to which the communication lines from the first communication circuit are connected, a second communication circuit (transmitting circuit 280, receiving circuit 300) connected to the pair of terminals and communicating with the first communication circuit, a switch 234 for setting the first communication circuit and the second communication circuit to a connected state or a disconnection state, a detection circuit (photocoupler 233) for detecting a voltage appearing between the pair of terminals, and a control circuit (control portion 310) for setting the switch to the disconnection state because occurrence of faulty wiring is estimated when the voltage detected by the detection circuit is equal to a predetermined threshold value or more, or setting the switch to the connection state in the other cases.