Provided is a screw compressor including a screw rotor configured to compress a gas due to rotation of the screw rotor about an axis of the screw rotor and a casing housing the screw rotor rotatably and provided with a suction port for a gas, the casing being provided with a suction side space through which a gas flowing into the casing from the suction port and not yet sucked by the screw rotor flows. The casing is provided with a heating fluid passage for introducing a heating fluid into the suction side space so as to heat oil staying in the suction side space.