An air conditioner includes a compressor, a four-way valve, an indoor heat exchanger, an expansion valve, an outdoor heat exchanger which are connected via pipings to form a heat pump type refrigerant circuit, the four-way valve being switched over to select a heating or cooling mode of operation. This air conditioner is further provided with a first and second branch pipes which extend from an outlet conduit of the compressor, the first branch pipe forming a first bypass pipe connected to an inlet conduit of the compressor. The first and second branch pipes have first and second valves, respectively. The air conditioner is also provided with a microcomputer for selectively opening and closing the second valve in response to the degree of superheating of discharged gaseous refrigerant after the first valve has been opened so that the discharged gaseous refrigerant is supplied to the outdoor heat exchanger to effect a defrosting operation while a heating operation is continued. When the degree of superheating of discharged gaseous refrigerant is low, a part of discharged gas is supplied to the inlet conduit so as to regulate the degree of superheating.