A screw compressor comprising: an electric motor configured to operate at a variable rotation speed; a screw rotor including a plurality of screw grooves on an outer circumferential surface; a screw shaft configured to transmit a driving force of the electric motor to the screw rotor and rotate the screw rotor; a first gate rotor and a second gate rotor, the first gate rotor being disposed on one side of the screw rotor, the second gate rotor being disposed on an other side of the screw rotor, the first gate rotor and the second gate rotor being disposed point-symmetrically with respect to the screw shaft and including, in outer circumferential portions, a plurality of teeth meshed with the screw grooves; a casing having a cylindrical shape to house the screw rotor inside a cylinder; the first gate rotor, the screw groove and the casing forming, by surrounding, a first compression chamber, the second gate rotor, the screw groove and the casing forming, by surrounding, a second compression chamber, a bypass device including a first slide valve, which is provided only on the first compression chamber of the two compression chambers (5b, 5c) and configured to slide in the direction of the rotational axis of the screw rotor and to allow the first compression chamber and a low-pressure chamber to communicate with each other, the low-pressure chamber being configured to have atmosphere of a suction pressure; and an economizer port being provided to the casing only on the first compression chamber side to communicate with the first compression chamber and configured to allow fluid flowing from an outside to flow inside the casing, the screw compressor is configured to, during a low-load operation, move the first slide valve to a position where the first compression chamber comes into a no-load state in which a pressure of the fluid in the first compression chamber does not rise, while the screw compressor 102 is driven with an increased rotation speed. an electric motor configured to operate at a variable rotation speed; a screw rotor including a plurality of screw grooves on an outer circumferential surface; a screw shaft configured to transmit a driving force of the electric motor to the screw rotor and rotate the screw rotor; a first gate rotor and a second gate rotor, the first gate rotor being disposed on one side of the screw rotor, the second gate rotor being disposed on an other side of the screw rotor, the first gate rotor and the second gate rotor being disposed point-symmetrically with respect to the screw shaft and including, in outer circumferential portions, a plurality of teeth meshed with the screw grooves; a casing having a cylindrical shape to house the screw rotor inside a cylinder; the first gate rotor, the screw groove and the casing forming, by surrounding, a first compression chamber, the second gate rotor, the screw groove and the casing forming, by surrounding, a second compression chamber, a bypass device including a first slide valve, which is provided only on the first compression chamber of the two compression chambers (5b, 5c) and configured to slide in the direction of the rotational axis of the screw rotor and to allow the first compression chamber and a low-pressure chamber to communicate with each other, the low-pressure chamber being configured to have atmosphere of a suction pressure; and an economizer port being provided to the casing only on the first compression chamber side to communicate with the first compression chamber and configured to allow fluid flowing from an outside to flow inside the casing, the screw compressor is configured to, during a low-load operation, move the first slide valve to a position where the first compression chamber comes into a no-load state in which a pressure of the fluid in the first compression chamber does not rise, while the screw compressor 102 is driven with an increased rotation speed.