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 a circumferential direction on an inner wall surface of the outer cylinder, wherein an 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, characterised in that the outer cylinder includes a guide portion, the guide portion comprises a side wall configured to gradually reduce a swirling radius of the refrigerant swirling in the guide portion, and an upper wall and a lower wall for suppressing the refrigerant swirling in the guide portion from spreading in a perpendicular direction, wherein the refrigerant flowing into the guide portion through the introducing flow path swirls while suppressed from spreading in the perpendicular direction in the guide portion and then is discharged from the guide portion so as to swirl and go down along the inner wall surface of the outer cylinder, thereby separated into the refrigerant gas and the oil, wherein the separated refrigerant gas flows into the inner cylinder from a lower end portion of the inner cylinder and goes up while the separated oil flown down along the inner wall surface of the outer cylinder. 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 a circumferential direction on an inner wall surface of the outer cylinder, wherein an 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, characterised in that the outer cylinder includes a guide portion, the guide portion comprises a side wall configured to gradually reduce a swirling radius of the refrigerant swirling in the guide portion, and an upper wall and a lower wall for suppressing the refrigerant swirling in the guide portion from spreading in a perpendicular direction, wherein the refrigerant flowing into the guide portion through the introducing flow path swirls while suppressed from spreading in the perpendicular direction in the guide portion and then is discharged from the guide portion so as to swirl and go down along the inner wall surface of the outer cylinder, thereby separated into the refrigerant gas and the oil, wherein the separated refrigerant gas flows into the inner cylinder from a lower end portion of the inner cylinder and goes up while the separated oil flown down along the inner wall surface of the outer cylinder.