An air conditioner including: a) an outdoor unit having a1) a compressor, a2) an outdoor heat exchanger (23, 23a, 23b), a3) an outdoor liquid-refrigerant pipe that connects a liquid-side end of the outdoor heat exchanger (23, 23a, 23b) to a connection portion of the outdoor unit; a4) a liquid-pressure adjustment expansion valve provided in the outdoor liquid-refrigerant pipe that decompresses the refrigerant during cooling operation so that refrigerant is in a gas-liquid two phase state during cooling operation; a5) an outdoor expansion valve that decompresses the refrigerant during heating operation which is placed between the liquid-pressure adjustment expansion valve and the outdoor heat exchanger (23, 23a, 23b) in the outdoor liquid-refrigerant pipe; b) a plurality of indoor units (3a, 3b, 3c, 3d) having an indoor heat exchanger (52a, 52b, 52c, 52d) and an indoor expansion valve (51a, 51b, 51c, 51d), and c) a liquid-refrigerant connection pipe that connects the connection portion the outdoor unit to the plurality of indoor units (3a, 3b, 3c, 3d), d) the air conditioner being configured to perform an operation of sending a refrigerant, which has been discharged from the compressor, to the outdoor heat exchanger (23, 23a, 23b), outdoor liquid-refrigerant pipe, the liquid-refrigerant connection pipe, and the indoor heat exchanger (52a, 52b, 52c, 52d) in that order, e) a switching mechanism (22, 22a, 22b) that switches the flow of refrigerant to connect the discharge side of the compressor and the gas side of the outdoor heat exchanger (23, 23a, 23b) when the outdoor heat exchanger (23, 23a, 23b) functions as a radiator of the refrigerant and to connect the suction side of the compressor and the gas side of the outdoor heat exchanger (23, 23a, 23b) when the outdoor heat exchanger (23, 23a, 23b) functions as an evaporator of the refrigerant; and f) a liquid injection pipe that is connected to at least one part of an outlet side of an accumulator, an inlet side of the accumulator and an intermediate portion of a compression stroke of the compressor so as to branch part of the refrigerant flowing through the outdoor liquid-refrigerant pipe and to feed the branched refrigerant to the compressor during cooling operation. a) an outdoor unit having a1) a compressor, a2) an outdoor heat exchanger (23, 23a, 23b), a3) an outdoor liquid-refrigerant pipe that connects a liquid-side end of the outdoor heat exchanger (23, 23a, 23b) to a connection portion of the outdoor unit; a4) a liquid-pressure adjustment expansion valve provided in the outdoor liquid-refrigerant pipe that decompresses the refrigerant during cooling operation so that refrigerant is in a gas-liquid two phase state during cooling operation; a5) an outdoor expansion valve that decompresses the refrigerant during heating operation which is placed between the liquid-pressure adjustment expansion valve and the outdoor heat exchanger (23, 23a, 23b) in the outdoor liquid-refrigerant pipe; a1) a compressor, a2) an outdoor heat exchanger (23, 23a, 23b), a3) an outdoor liquid-refrigerant pipe that connects a liquid-side end of the outdoor heat exchanger (23, 23a, 23b) to a connection portion of the outdoor unit; a4) a liquid-pressure adjustment expansion valve provided in the outdoor liquid-refrigerant pipe that decompresses the refrigerant during cooling operation so that refrigerant is in a gas-liquid two phase state during cooling operation; a5) an outdoor expansion valve that decompresses the refrigerant during heating operation which is placed between the liquid-pressure adjustment expansion valve and the outdoor heat exchanger (23, 23a, 23b) in the outdoor liquid-refrigerant pipe; b) a plurality of indoor units (3a, 3b, 3c, 3d) having an indoor heat exchanger (52a, 52b, 52c, 52d) and an indoor expansion valve (51a, 51b, 51c, 51d), and c) a liquid-refrigerant connection pipe that connects the connection portion the outdoor unit to the plurality of indoor units (3a, 3b, 3c, 3d), d) the air conditioner being configured to perform an operation of sending a refrigerant, which has been discharged from the compressor, to the outdoor heat exchanger (23, 23a, 23b), outdoor liquid-refrigerant pipe, the liquid-refrigerant connection pipe, and the indoor heat exchanger (52a, 52b, 52c, 52d) in that order, e) a switching mechanism (22, 22a, 22b) that switches the flow of refrigerant to connect the discharge side of the compressor and the gas side of the outdoor heat exchanger (23, 23a, 23b) when the outdoor heat exchanger (23, 23a, 23b) functions as a radiator of the refrigerant and to connect the suction side of the compressor and the gas side of the outdoor heat exchanger (23, 23a, 23b) when the outdoor heat exchanger (23, 23a, 23b) functions as an evaporator of the refrigerant; and f) a liquid injection pipe that is connected to at least one part of an outlet side of an accumulator, an inlet side of the accumulator and an intermediate portion of a compression stroke of the compressor so as to branch part of the refrigerant flowing through the outdoor liquid-refrigerant pipe and to feed the branched refrigerant to the compressor during cooling operation.