A rotary compressor comprising: a shell defining an internal space; a driving motor provided inside the internal space of the shell; and a compression mechanism unit configured to compress refrigerant and to be powered by the driving motor, wherein the compression mechanism unit comprises: a cylinder having a chamber for compressing refrigerant therein; a rotary shaft connected to the driving motor; a roller provided in the chamber and connected to the rotary shaft for compressing refrigerant in the chamber; a bearing coupled to the cylinder and having a discharge port for allowing refrigerant compressed in the chamber to pass therethrough; a muffler (62, 72) coupled to the bearing for receiving refrigerant that has passed through the discharge port; and a noise reducing unit (65, 75) attached to the muffler (62, 72) and configured to define a noise reducing chamber together with the muffler (62, 72), characterized in that the noise reducing unit (65, 75) comprises a chamber defining body (651, 751) defining at least a portion of the noise reducing chamber, and wherein the chamber defining body (651, 751) is coupled to the muffler (62, 72), wherein the chamber defining body (651, 751) comprises opposite side surfaces and an upper surface such that a vertical section of the chamber defining body has a shape of "Π", wherein the "II" shaped chamber defining body is spaced apart from the rotary shaft and surrounds a circumference of the rotary shaft, the "Π" shaped chamber defining body forming a plurality of concave portions and a plurality of convex portions when viewed from above along the rotational axis direction, with the concave portions and the convex portions being alternately arranged along a length of the "Π" shaped chamber defining body. a shell defining an internal space; a driving motor provided inside the internal space of the shell; and a compression mechanism unit configured to compress refrigerant and to be powered by the driving motor, wherein the compression mechanism unit comprises: a cylinder having a chamber for compressing refrigerant therein; a rotary shaft connected to the driving motor; a roller provided in the chamber and connected to the rotary shaft for compressing refrigerant in the chamber; a bearing coupled to the cylinder and having a discharge port for allowing refrigerant compressed in the chamber to pass therethrough; a muffler (62, 72) coupled to the bearing for receiving refrigerant that has passed through the discharge port; and a noise reducing unit (65, 75) attached to the muffler (62, 72) and configured to define a noise reducing chamber together with the muffler (62, 72), characterized in that the noise reducing unit (65, 75) comprises a chamber defining body (651, 751) defining at least a portion of the noise reducing chamber, and wherein the chamber defining body (651, 751) is coupled to the muffler (62, 72), wherein the chamber defining body (651, 751) comprises opposite side surfaces and an upper surface such that a vertical section of the chamber defining body has a shape of "Π", wherein the "II" shaped chamber defining body is spaced apart from the rotary shaft and surrounds a circumference of the rotary shaft, the "Π" shaped chamber defining body forming a plurality of concave portions and a plurality of convex portions when viewed from above along the rotational axis direction, with the concave portions and the convex portions being alternately arranged along a length of the "Π" shaped chamber defining body. a cylinder having a chamber for compressing refrigerant therein; a rotary shaft connected to the driving motor; a roller provided in the chamber and connected to the rotary shaft for compressing refrigerant in the chamber; a bearing coupled to the cylinder and having a discharge port for allowing refrigerant compressed in the chamber to pass therethrough; a muffler (62, 72) coupled to the bearing for receiving refrigerant that has passed through the discharge port; and a noise reducing unit (65, 75) attached to the muffler (62, 72) and configured to define a noise reducing chamber together with the muffler (62, 72), characterized in that the noise reducing unit (65, 75) comprises a chamber defining body (651, 751) defining at least a portion of the noise reducing chamber, and wherein the chamber defining body (651, 751) is coupled to the muffler (62, 72), wherein the chamber defining body (651, 751) comprises opposite side surfaces and an upper surface such that a vertical section of the chamber defining body has a shape of "Π", wherein the "II" shaped chamber defining body is spaced apart from the rotary shaft and surrounds a circumference of the rotary shaft, the "Π" shaped chamber defining body forming a plurality of concave portions and a plurality of convex portions when viewed from above along the rotational axis direction, with the concave portions and the convex portions being alternately arranged along a length of the "Π" shaped chamber defining body.