A screw compressor comprising: a male rotor and a female rotor whose rotating shafts are substantially parallel to each other and which are rotated in engagement with each other, forming a compression working chamber; and an oil separator into which refrigerant discharged from the compression working chamber flows and which separates the refrigerant into refrigerant gas and oil, wherein the oil separator includes: an outer cylinder; an inner cylinder located inside the outer cylinder; and an introducing flow path letting refrigerant discharged from the compression working chamber flow in so that the refrigerant swirls to the circumferential direction on the inner wall surface of the outer cylinder, wherein the upper end portion of the inner cylinder is held by the oil separator and the outer wall of the inner cylinder is thereby held independently of the inner wall of the outer cylinder, wherein the diameter of the inner wall of the outer cylinder is so configured that the diameter is gradually reduced along the swirl flow path in which refrigerant that flowed in through the introducing flow path swirls, and wherein refrigerant that flowed in through the introducing flow path swirls and goes down along the inner wall of the outer cylinder and is thereby separated into gas refrigerant and oil, thereafter the separated gas refrigerant flowing from the lower end portion of the inner cylinder into the inner cylinder and going up and the separated oil flowing down along the inner wall surface of the outer cylinder, characterized in that an inclined portion in which the diameter of the inner wall of the outer cylinder is increased as it goes downward from the swirl flow path is provided. a male rotor and a female rotor whose rotating shafts are substantially parallel to each other and which are rotated in engagement with each other, forming a compression working chamber; and an oil separator into which refrigerant discharged from the compression working chamber flows and which separates the refrigerant into refrigerant gas and oil, wherein the oil separator includes: an outer cylinder; an inner cylinder located inside the outer cylinder; and an introducing flow path letting refrigerant discharged from the compression working chamber flow in so that the refrigerant swirls to the circumferential direction on the inner wall surface of the outer cylinder, wherein the upper end portion of the inner cylinder is held by the oil separator and the outer wall of the inner cylinder is thereby held independently of the inner wall of the outer cylinder, wherein the diameter of the inner wall of the outer cylinder is so configured that the diameter is gradually reduced along the swirl flow path in which refrigerant that flowed in through the introducing flow path swirls, and wherein refrigerant that flowed in through the introducing flow path swirls and goes down along the inner wall of the outer cylinder and is thereby separated into gas refrigerant and oil, thereafter the separated gas refrigerant flowing from the lower end portion of the inner cylinder into the inner cylinder and going up and the separated oil flowing down along the inner wall surface of the outer cylinder, characterized in that an inclined portion in which the diameter of the inner wall of the outer cylinder is increased as it goes downward from the swirl flow path is provided.