A screw compressor includes a screw rotor, a gate rotor, a drive mechanism to rotate the screw rotor, a cylinder, a discharge port and an adjustment mechanism. The screw rotor has an outer periphery with a helical groove engaged with radially arranged gates of the gate rotor. First and second axial ends of the screw rotor form suction and discharge sides, respectively. The cylinder accommodates the screw rotor to define a compression chamber in the helical groove. Fluid in the compression chamber flows through the discharge port toward the discharge side of the screw rotor. The adjustment mechanism is configured to adjust a compression ratio of the compression chamber within a predetermined range. The adjustment mechanism adjusts the compression ratio to a minimum compression ratio immediately before or when operation of the adjustment mechanism is stopped.