A single screw compressor in which an axial load acting on the screw rotor can be reduced. The single screw compressor (1) comprises a screw rotor (2) and a casing (3). The screw rotor (2) has a plurality of spiral grooves (6) in the outer circumferential surface and has such a tapered shape as the outside diameter increases from the suction side toward the delivery side. The casing (3) encases the screw rotor (2). The screw rotor (2) has a reversely tapered portion (8). On the outer circumferential surface having the spiral grooves (6), outside diameter of the reversely tapered portion (8) decreases from the maximum outside diameter portion (B) on the downstream side of the delivery side maximum outside diameter portion (B).