An air conditioning system comprising an outdoor and an indoor unit connected through common power supply lines. The outdoor unit including a first communication circuit using one of the plurality of common power supply lines and one signal line independent of the power supply lines. The indoor unit comprising terminals connected to the first communication circuit; a switch configured to set the first and second communication circuits to a connection state or a disconnection state; a detection circuit configured to detect a voltage between the terminals; and a controller configured to control the switch. The switch sets the first and second communication circuits to the disconnection state when the voltage detected by the detection circuit equals or exceeds a predetermined threshold value; otherwise the switch sets the first and second communication circuits to the connection state.