An air conditioner comprising an outdoor device and an indoor device that are connected to each other via a three-core cable including a power line, a power-signal common line, and a signal line, three-phase AC power being connected to the outdoor device via a power supply relay, and single-phase AC power being supplied to the indoor device via the power line and the power-signal common line, which are two connection lines connected to the three-phase AC power source, wherein the indoor device includes an outdoor activation relay that opens and closes connection between the power line and the power-signal common line, and an indoor control unit that operates the outdoor activation relay in order to supply single-phase AC power between the signal line and the power-signal common line, the outdoor device includes an outdoor rectifier unit that is connected to the power supply relay, a smoothing capacitor that smooths an output of the outdoor rectifier unit, an outdoor control unit that is operated by a voltage of the smoothing capacitor, a power-supply switching relay to which the signal line is connected, an inrush-current preventing-relay drive unit that is operated when single-phase AC power is supplied between the power-signal common line and the signal line via the outdoor activation relay and the power-supply switching relay, a first inrush-current preventing relay that is connected in parallel with the power supply relay and is controlled by the inrush-current preventing-relay drive unit, characterized by: a second inrush-current preventing relay that is connected in parallel with the first inrush-current preventing relay and is controlled by the outdoor control unit, and wherein the second inrush-current preventing relay is closed when an external power supply is interrupted. three-phase AC power being connected to the outdoor device via a power supply relay, and single-phase AC power being supplied to the indoor device via the power line and the power-signal common line, which are two connection lines connected to the three-phase AC power source, wherein an outdoor activation relay that opens and closes connection between the power line and the power-signal common line, and an indoor control unit that operates the outdoor activation relay in order to supply single-phase AC power between the signal line and the power-signal common line, an outdoor rectifier unit that is connected to the power supply relay, a smoothing capacitor that smooths an output of the outdoor rectifier unit, an outdoor control unit that is operated by a voltage of the smoothing capacitor, a power-supply switching relay to which the signal line is connected, an inrush-current preventing-relay drive unit that is operated when single-phase AC power is supplied between the power-signal common line and the signal line via the outdoor activation relay and the power-supply switching relay, a first inrush-current preventing relay that is connected in parallel with the power supply relay and is controlled by the inrush-current preventing-relay drive unit, characterized by: a second inrush-current preventing relay that is connected in parallel with the first inrush-current preventing relay and is controlled by the outdoor control unit, and wherein