A slide valve (70) is arranged on the side of a screw rotor (40) in a casing of a screw compressor. A bypass passage (33) is formed in the casing to connect a fluid chamber (23) to a low pressure space. When the slide valve (70) slides, the size of an opening (34) of the bypass passage (33) in an inner peripheral surface (35) of a cylindrical wall (30) changes whereby the operating capacity of the screw compressor changes. A front end surface (P2) of the slide valve (70) is inclined along an extension direction of a spiral groove (41) of the screw rotor (40). Also, a seat surface (P1) of the cylindrical wall (30) opposed to the front end surface (P2) of the slide valve (70) is parallel with the front end surface (P2) of the slide valve (70).
