merges in the pipe 14 the other pipe 21 is connected through a check valve 151 to a pipe 22 which merges in the pipe 16 the gas injection passage 90 has one end opening to the gaseous phase part 51 of the gas liquid separator 50 and the other end connected to the compression chamber of the compressor 10 in the cooling operation of the heat pump the four way valve 120 takes a position as illustrated by full line in fig5 consequently the refrigerant flows in the direction indicated by full line arrows on the other hand for heating operation of the heat pump the four way valve is switched to the position shown by broken line so that the refrigerant flows as indicated by broken line arrows the cooling operation of the embodiment of fig5 with gas injection is as follows the solenoid actuated valves 103 and 40 are opened while the solenoid actuated valve 105 is closed in accordance with the instructions given by a controller 56 responsive to a sensor 55 sensitive to the air temperature in a room the refrigerant discharged from the compressor 10 is recycled to the latter through a circuit which is constituted by the four way valve 120 the pipe 11 the outdoor heat exchanger 130 the pipe 13 the check valve 132 the pipe 14 the first pressure reducer 102 for cooling the solenoid.