A screw compressor, comprising: a first rotor provided with a helical groove; a second rotor which meshes with the first rotor and rotates together with the first rotor; a rotor casing which covers at least an outer periphery of the first rotor, and defines a compression chamber in the helical groove together with the first rotor and the second rotor, wherein a fluid is compressed in the compression chamber, at least one of the first rotor and the second rotor is provided with an oil supply passage which is connected to an oil supply port opened at a sliding surface of the at least one rotor (40, 50) to supply a lubricant to the sliding surface, the first rotor is a screw rotor rotatably housed in a cylindrical wall constituting the rotor casing, the second rotor is a gear-shaped gate rotor having a plurality of flat gates and arranged outside the cylindrical wall, some of the gates entering a space inside the cylindrical wall via an opening formed in the cylindrical wall and meshing with the screw rotor so that the gate rotor rotates together with the screw rotor, characterised in that the oil supply passage is formed in at least one of the gates of the gate rotor, the oil supply port is a lateral oil supply port opened at a side surface (51a, 51b) of the at least one gate, the side surface (51a, 51b) serving as the sliding surface which slides on the screw rotor, and the lateral oil supply port is opened, in the thickness direction of the at least one gate, at a position closer to the compression chamber than the seal line (L1, L2) which abuts on the corresponding lateral face (42a, 42b) of the helical groove of the screw rotor of the at least one gate. a first rotor provided with a helical groove; a second rotor which meshes with the first rotor and rotates together with the first rotor; a rotor casing which covers at least an outer periphery of the first rotor, and defines a compression chamber in the helical groove together with the first rotor and the second rotor, wherein a fluid is compressed in the compression chamber, at least one of the first rotor and the second rotor is provided with an oil supply passage which is connected to an oil supply port opened at a sliding surface of the at least one rotor (40, 50) to supply a lubricant to the sliding surface, the first rotor is a screw rotor rotatably housed in a cylindrical wall constituting the rotor casing, the second rotor is a gear-shaped gate rotor having a plurality of flat gates and arranged outside the cylindrical wall, some of the gates entering a space inside the cylindrical wall via an opening formed in the cylindrical wall and meshing with the screw rotor so that the gate rotor rotates together with the screw rotor, characterised in that the oil supply passage is formed in at least one of the gates of the gate rotor, the oil supply port is a lateral oil supply port opened at a side surface (51a, 51b) of the at least one gate, the side surface (51a, 51b) serving as the sliding surface which slides on the screw rotor, and the lateral oil supply port is opened, in the thickness direction of the at least one gate, at a position closer to the compression chamber than the seal line (L1, L2) which abuts on the corresponding lateral face (42a, 42b) of the helical groove of the screw rotor of the at least one gate.