A screw compressor has a balance bearing whose outer ring is movable relative to the casing. A balance piston divides a balance cylinder into a high pressure fluid chamber and a low pressure fluid chamber, and movement of the piston in the balance cylinder is transmitted to the outer ring of the balance bearing. The high pressure fluid chamber and the low pressure fluid chamber are arranged such that higher pressure fluid in the high pressure fluid chamber urges the piston to press the outer ring of the balance bearing oppositely to a spring and toward the main thrust bearing, and lower pressure fluid in the low pressure fluid chamber urges the piston to press the outer ring of the balance bearing away from the main thrust bearing.