A compressor unit comprising: a compressor ; a condenser ; and an injection pipe constituting a part of a refrigerant pipe connecting the compressor and the condenser , the injection pipe permitting a part of refrigerant flowing out of the condenser to branch off and flow into the compressor , wherein the injection pipe includes: a compressor connecting part connected to the compressor ; an expansion valve connecting part connected to an expansion valve disposed in the injection pipe ; and an injection muffler disposed between the compressor connecting part and the expansion valve connecting part , characterized in that the injection muffler has an inner diameter larger than an inner diameter of the compressor connecting part and an inner diameter of the expansion valve connecting part , and satisfies a relationship of 3 ≤ Vrat ≤ 5, where Vst is a compressor displacement volume, Vmuff is an injection muffler volume, and Vrat is a ratio of the injection muffler volume to the compressor displacement volume (Vmuff / Vst), and the injection muffler is fixed to a side of the compressor , wherein the injection pipe includes a distal end continuous to the compressor connecting part and disposed inside the compressor , the compressor includes an injection port through which the refrigerant flowing into the compressor via the distal end of the injection pipe is led into a compression chamber of the compressor , the injection pipe satisfies relationships of φport ≤ φinj ≤ φd1 ≤ φmuff and φinj ≤ φd2 ≤ φmuff, where φport is an inner diameter of the injection port , φinj is an inner diameter of the distal end , φd1 is the inner diameter of the compressor connecting part , φd2 is the inner diameter of the expansion valve connecting part and φmuff is the inner diameter of the injection muffler . a compressor ; a condenser ; and an injection pipe constituting a part of a refrigerant pipe connecting the compressor and the condenser , the injection pipe permitting a part of refrigerant flowing out of the condenser to branch off and flow into the compressor , wherein the injection pipe includes: a compressor connecting part connected to the compressor ; an expansion valve connecting part connected to an expansion valve disposed in the injection pipe ; and an injection muffler disposed between the compressor connecting part and the expansion valve connecting part , a compressor connecting part connected to the compressor ; an expansion valve connecting part connected to an expansion valve disposed in the injection pipe ; and an injection muffler disposed between the compressor connecting part and the expansion valve connecting part , characterized in that the injection muffler has an inner diameter larger than an inner diameter of the compressor connecting part and an inner diameter of the expansion valve connecting part , and satisfies a relationship of 3 ≤ Vrat ≤ 5, where Vst is a compressor displacement volume, Vmuff is an injection muffler volume, and Vrat is a ratio of the injection muffler volume to the compressor displacement volume (Vmuff / Vst), and the injection muffler is fixed to a side of the compressor , wherein the injection pipe includes a distal end continuous to the compressor connecting part and disposed inside the compressor , the compressor includes an injection port through which the refrigerant flowing into the compressor via the distal end of the injection pipe is led into a compression chamber of the compressor , the injection pipe satisfies relationships of φport ≤ φinj ≤ φd1 ≤ φmuff and φinj ≤ φd2 ≤ φmuff, where φport is an inner diameter of the injection port , φinj is an inner diameter of the distal end , φd1 is the inner diameter of the compressor connecting part , φd2 is the inner diameter of the expansion valve connecting part and φmuff is the inner diameter of the injection muffler . <EOS>