A single screw compressor structure includes a screw rotor and a casing. The screw rotor has a plurality of helical grooves formed in an outer peripheral surface thereof. The casing houses the screw rotor. The screw rotor includes a main tapered portion having a tapered outer diameter that becomes larger from an intake side toward a discharge side of the screw rotor, and a reversely tapered portion that is located on a downstream side of a maximum outer diameter portion of the outer surface and on the discharge side of the main tapered portion. The reversely tapered portion has a reversely tapered outer diameter that becomes smaller as the reversely tapered portion extends away from the maximum outer diameter portion.