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.