A screw compressor comprising: a casing body that has, inside the casing body, a high-pressure space and a low-pressure space; a screw rotor that has a plurality of grooves that are each shaped in a spiral and are formed at an outer circumferential surface of the screw rotor, the screw rotor being rotationally driven; gate rotors, each of which has a plurality of gate-rotor tooth portions to mesh with the plurality of grooves of the screw rotor, the gate rotors defining a compression chamber together with the casing body and the screw rotor; a slide valve accommodated in a slide groove formed in an inner wall surface of the casing body, the slide valve being configured to move such that the slide valve slides in a rotational-shaft direction of the screw rotor; and a slide valve movement mechanism configured to move the slide valve such that the slide valve slides in the rotational-shaft direction of the screw rotor, the slide valve movement mechanism having a cylinder provided in the casing body, the cylinder being hollow, a piston connected with the slide valve, the piston partitioning an interior of the cylinder into a first chamber and a second chamber, a communication flow passage through which the second chamber communicates with the low-pressure space, and a valve configured to open and close the communication flow passage, the slide valve movement mechanism being configured to vary a pressure in the second chamber by opening and closing the valve and thus move the piston and the slide valve, the cylinder being provided with a first inflow hole, a second inflow hole, and a third inflow hole, the first chamber communicating with the high-pressure space through the first inflow hole, the second chamber communicating with the low-pressure space through the second inflow hole and the communication flow passage, the second chamber communicating with the high-pressure space through the third inflow hole, the third inflow hole being located at a position at which the third inflow hole is closed by the piston when the piston lies at a stop position at which the piston moves toward the second chamber and stops. a casing body that has, inside the casing body, a high-pressure space and a low-pressure space; a screw rotor that has a plurality of grooves that are each shaped in a spiral and are formed at an outer circumferential surface of the screw rotor, the screw rotor being rotationally driven; gate rotors, each of which has a plurality of gate-rotor tooth portions to mesh with the plurality of grooves of the screw rotor, the gate rotors defining a compression chamber together with the casing body and the screw rotor; a slide valve accommodated in a slide groove formed in an inner wall surface of the casing body, the slide valve being configured to move such that the slide valve slides in a rotational-shaft direction of the screw rotor; and a slide valve movement mechanism configured to move the slide valve such that the slide valve slides in the rotational-shaft direction of the screw rotor, the slide valve movement mechanism having a cylinder provided in the casing body, the cylinder being hollow, a piston connected with the slide valve, the piston partitioning an interior of the cylinder into a first chamber and a second chamber, a communication flow passage through which the second chamber communicates with the low-pressure space, and a valve configured to open and close the communication flow passage, the slide valve movement mechanism being configured to vary a pressure in the second chamber by opening and closing the valve and thus move the piston and the slide valve, the cylinder being provided with a first inflow hole, a second inflow hole, and a third inflow hole, the first chamber communicating with the high-pressure space through the first inflow hole, the second chamber communicating with the low-pressure space through the second inflow hole and the communication flow passage, the second chamber communicating with the high-pressure space through the third inflow hole, the third inflow hole being located at a position at which the third inflow hole is closed by the piston when the piston lies at a stop position at which the piston moves toward the second chamber and stops.