A screw compressor includes a casing having low and high pressure spaces, a screw rotor inserted in a cylinder part of the casing, and a slide valve disclosed in the cylinder part. The screw rotor has a plurality of helical grooves forming a compression chamber. The slide valve is moveable along an axis of the screw rotor and faces an outer periphery of the screw rotor to form a discharge port to communicating the compression chamber with the high pressure space. Fluid in the low pressure space is sucked into the compression chamber, compressed, and then discharged to the high-pressure space when the screw rotor rotates. The slide valve includes a sealing projection located on a back surface of the slide valve opposite to the screw rotor, and separating the low and high pressure spaces from each other when the sealing projection is in slidable contact with the casing.