To realize comfortable air conditioning by improving the heat exchanging capacity of an outdoor heat exchanger and drastically shortening defrosting time.

An air conditioner comprises, during heating operation, a refrigerant channel, which consists of a compressor 1, a four-way valve 2, an indoor heat exchanger 5, an expansion valve 4 and an outdoor heat exchanger 3, and comprises a hot water circulation path 11 with a medium being sealed therein, which is formed by successively connecting with pipes a circulating pump 6, a heat receiver 7, a heat storage tank 8, a first on-off valve 9, a heat transfer device and a check valve 10. In defrosting operation, there are formed the refrigerant channel consisting of the compressor, the four-way valve, the indoor heat exchanger, the expansion valve and the outdoor heat exchanger, and the hot water circulation path consisting of the circulating pump, the heat receiver, the heat storage tank, the first on-off valve, the heat transfer device, the check valve and a side path 13 which comprises a second on-off valve 12.