A screw compressor comprising a screw rotor, a gate rotor that meshes with the screw rotor, a cylindrical wall into which the screw rotor is rotatably inserted, and a slide valve that adjusts an opening area of an opening formed in the cylindrical wall, wherein the slide valve includes a valve body and a guide portion, wherein the valve body extends in an axial direction of the cylindrical wall and has a crescent shape in a cross section in a perpendicular direction that is perpendicular to the axial direction, a radius of curvature of an inner arc-like curved surface of the crescent shape is substantially equal to a radius of curvature of an inner peripheral surface of the cylindrical wall, and a radius of curvature of an outer arc-like curved surface of the crescent shape is smaller than the radius of curvature of the inner arc-like curved surface, and a central angle (θ) of the outer arc-like curved surface is smaller than or equal to 180°, wherein the guide portion is configured to allow movement of the valve body in the axial direction and restrict movement of the valve body in the perpendicular direction, and wherein the guide portion has a cylindrical shape and a center thereof is disposed at a position that is displaced from a center of curvature of the outer arc-like curved surface of the valve body, the valve body has a thickness along a line connecting a center of the outer arc-like curved surface and a center of the inner arc-like curved surface, the screw compressor is characterised in that the dimension is smaller than a diameter of the guide portion, and a position of an outer end of the arc-like curved surface and a position of an outer end of the outer peripheral surface of the guide portion in the radial direction of the screw rotor are the same wherein the slide valve includes a valve body and a guide portion, wherein the valve body extends in an axial direction of the cylindrical wall and has a crescent shape in a cross section in a perpendicular direction that is perpendicular to the axial direction, a radius of curvature of an inner arc-like curved surface of the crescent shape is substantially equal to a radius of curvature of an inner peripheral surface of the cylindrical wall, and a radius of curvature of an outer arc-like curved surface of the crescent shape is smaller than the radius of curvature of the inner arc-like curved surface, and a central angle (θ) of the outer arc-like curved surface is smaller than or equal to 180°, wherein the guide portion is configured to allow movement of the valve body in the axial direction and restrict movement of the valve body in the perpendicular direction, and wherein the guide portion has a cylindrical shape and a center thereof is disposed at a position that is displaced from a center of curvature of the outer arc-like curved surface of the valve body, the valve body has a thickness along a line connecting a center of the outer arc-like curved surface and a center of the inner arc-like curved surface, the screw compressor is characterised in that the dimension is smaller than a diameter of the guide portion, and a position of an outer end of the arc-like curved surface and a position of an outer end of the outer peripheral surface of the guide portion in the radial direction of the screw rotor are the same