<p>A screw compressor (1) is provided with a slide valve (60) for changing the compression ratio. A sealing protrusion section (66) is formed on the valve element section (65) of the slide valve (60) so as to extend along the rear end surface (74) of the valve element section (65). In the slide valve containing section (31) of the casing (10), the sealing protrusion section (66) of the slide valve (60) is in sliding contact with the sliding contact curved surface (32) of the casing (10) to separate a low-pressure space (S1) and a high-pressure space (S2). The refrigerant pressure within the low-pressure space (S1) always acts on the entire non-contact surface (77) of the valve element section (65). This causes a force which presses the valve element section (65) toward the screw rotor (40) to be constant irrespective of the position of the slide valve (60), and as a result, a variation in the clearance between the front face (71) of the valve element section (65) and the screw rotor (40) is reduced.</p>
