To provide a heating strengthening air conditioner for improving capacity of heating operation, by dispensing with defrosting operation by preventing frosting in an outdoor heat exchanger in the heating operation at the low outside air temperature not more than zero degree or in a cold district.

While constituting a main cycle 1 by successively connecting a first compressor 3, a four-way valve 4, the outdoor heat exchanger 5, a first expansion valve 7, an inter-refrigerant heat exchanger 8 and an indoor heat exchanger 9, a sub-cycle 2 is constituted by successively connecting a second compressor 13, the outdoor heat exchanger 5, a second expansion valve 14 and the inter-refrigerant heat exchanger 8. The outdoor heat exchanger 5 is heated by a refrigerant circulating in the sub-cycle 2 in heating operation.