A single-screw compressor includes a casing, a screw rotor accommodated in the casing, a gate rotor, and a gate rotor supporting member rotatably supporting the gate rotor. The gate rotor includes a plurality of flat-plate-shaped gates formed in a radial pattern and meshing with a helical groove of the screw rotor. Fluid in a compression chamber defined by the screw rotor, the casing and the gates is compressed when the screw rotor is rotated. The gate rotor supporting member includes a gate supporting portion supporting each gate from a back surface side. The gate rotor and the gate rotor supporting member form a gate rotor assembly including a pressure introduction channel configured and arranged to introduce a fluid pressure on a front surface side of each gate into a gap between a back surface of the gate and the gate supporting portion.