To efficiently control the flow of refrigerant by using a tube with a groove, as a piping, between the delivery side of a compressor and an outdoor heat exchanger and between the compressor and an indoor heat exchanger to restrain a turbulent flow of refrigerant.

In a refrigerant circuit of the air conditioner, a compressor 1, four-way valve 2, outdoor heat exchanger 3, pressure reducing device 4, and indoor heat exchanger 5 are successively connected by pipes 6a, 6b having different diameters. In this case, pipes 6a, 6b between the delivery side of the compressor 1 and the exchanger 3 and between the compressor 1 and the exchanger 5, where a turbulent flow tends to occur in the refrigerant which circulates in a cooling or heating operation, are formed from a pipe 1 having therein a plurality of grooves 7a in the same direction as the flow of refrigerant, and pipes 6a between the exchanger 3 and the device 4 and between the device 4 and the exchanger 5, where a turbulent flow tends to occur not so frequently as in the delivery side of the compressor, also are formed from the pipe 7, to restrain a turbulent flow of refrigerant. And an ordinary pipe is used for a pipe 6b connecting the valve 2 and the compressor 1 since a turbulent flow does not occur in the pipe 6b.