A screw compressor includes screw and gate rotors, a cylindrical wall, and a slide valve including a valve body and a guide portion. 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 to the axial direction. A radius of curvature of an inner arc shaped curved surface of the crescent shape is substantially equal to a radius of curvature of an inner peripheral surface of the cylindrical wall. A radius of curvature of an outer arc shaped curved surface of the crescent shape is smaller than the radius of curvature of the inner arc shaped curved surface, and a central angle of the outer arc shaped curved surface is 180° or less. The guide portion allows movement of the valve body in the axial direction and restricts movement in the perpendicular direction.