A screw compressor includes a screw rotor, a plurality of gate rotors, a casing with a cylindrical wall, and an adjustment mechanism to adjust a pressure of a fluid in the casing. A plurality of compression chambers are formed inside the cylindrical wall by the screw rotor and the gates. The plurality of compression chambers include a first compression chamber in which the fluid introduced into the casing with a suction pressure is compressed to a pressure higher than the suction pressure, and a second compression chamber to which the fluid compressed in the first compression chamber is sent via an intermediate pressure space. The adjustment mechanism adjusts an intermediate pressure, which is a pressure of the fluid in the intermediate pressure space, in a two-stage operation in which the fluid is compressed in the first compression chamber and the second compression chamber.