A single-screw compressor, comprising: a screw rotor having, in an outer circumferential surface thereof, a helical groove whose one end serves as a suction side and the other end serves as a discharge side of a fluid; a cylinder wall in which the screw rotor is rotatably accommodated; a driving mechanism which drives the screw rotor at a variable rotational speed according to a load; and a slide valve which is provided in a slide groove formed in the cylinder wall, faces the outer circumferential surface of the screw rotor to be movable in an axial direction, and adjusts a discharge start position by being moved in the axial direction according to the rotational speed, whereby an operation state in which a compression ratio is highest is an operation state in which a load factor is 100%, a discharge side end of the slide valve is in a shape along a facing portion that is a portion of a land of the screw rotor to which portion the discharge side end faces in a part load operation state in which the load factor is a predetermined value less than 100%, characterized in that the cylinder wall is provided with a cylinder-side discharge opening which communicates, at a discharge side of the screw rotor, with a compression chamber formed in the helical groove of the screw rotor, the cylinder-side discharge opening includes a main discharge port whose opening shape is determined according to a position of the slide valve in a state of operation where the load factor is 100%, and the main discharge port being open in both of the state of operation where the load factor is 100% and a state of operation of the part load, without being closed by the slide valve, and allowing the fluid to be discharged, and an auxiliary discharge port (28b, 28c, 28d) whose opening shape is determined according to the position of the slide valve in the state of operation of the part load, the auxiliary discharge port (28b, 28c, 28d) being closed by the slide valve in the state of operation where the load factor is 100%, and opened from the slide valve in the state of operation of the part load and allowing the fluid to be discharged, and the auxiliary discharge port (28b, 28c, 28d) includes a plurality of auxiliary discharge ports (28b, 28c, and 28d) to correspond to states of operation of a plurality of part loads a screw rotor having, in an outer circumferential surface thereof, a helical groove whose one end serves as a suction side and the other end serves as a discharge side of a fluid; a cylinder wall in which the screw rotor is rotatably accommodated; a driving mechanism which drives the screw rotor at a variable rotational speed according to a load; and a slide valve which is provided in a slide groove formed in the cylinder wall, faces the outer circumferential surface of the screw rotor to be movable in an axial direction, and adjusts a discharge start position by being moved in the axial direction according to the rotational speed, whereby an operation state in which a compression ratio is highest is an operation state in which a load factor is 100%, a discharge side end of the slide valve is in a shape along a facing portion that is a portion of a land of the screw rotor to which portion the discharge side end faces in a part load operation state in which the load factor is a predetermined value less than 100%, characterized in that the cylinder wall is provided with a cylinder-side discharge opening which communicates, at a discharge side of the screw rotor, with a compression chamber formed in the helical groove of the screw rotor, the cylinder-side discharge opening includes a main discharge port whose opening shape is determined according to a position of the slide valve in a state of operation where the load factor is 100%, and the main discharge port being open in both of the state of operation where the load factor is 100% and a state of operation of the part load, without being closed by the slide valve, and allowing the fluid to be discharged, and an auxiliary discharge port (28b, 28c, 28d) whose opening shape is determined according to the position of the slide valve in the state of operation of the part load, the auxiliary discharge port (28b, 28c, 28d) being closed by the slide valve in the state of operation where the load factor is 100%, and opened from the slide valve in the state of operation of the part load and allowing the fluid to be discharged, and a main discharge port whose opening shape is determined according to a position of the slide valve in a state of operation where the load factor is 100%, and the main discharge port being open in both of the state of operation where the load factor is 100% and a state of operation of the part load, without being closed by the slide valve, and allowing the fluid to be discharged, and an auxiliary discharge port (28b, 28c, 28d) whose opening shape is determined according to the position of the slide valve in the state of operation of the part load, the auxiliary discharge port (28b, 28c, 28d) being closed by the slide valve in the state of operation where the load factor is 100%, and opened from the slide valve in the state of operation of the part load and allowing the fluid to be discharged, and the auxiliary discharge port (28b, 28c, 28d) includes a plurality of auxiliary discharge ports (28b, 28c, and 28d) to correspond to states of operation of a plurality of part loads