A single screw compressor structure includes a screw rotor and a casing, which houses the screw rotor. A tapered outer circumferential surface of the screw rotor has a plurality of helical grooves, with an outer diameter that increases from an inlet side toward a discharge side. The casing includes an outer tube member having a circular inner hole to form an interior, and an inner tube member is fixed to the interior of the outer tube member. The inner tube member has a tapered inner surface that opposes the tapered outer circumferential surface of the screw rotor. A single screw compressor further including a gate rotor is assembled by adjusting mesh between the screw rotor and the gate rotor, aligning the tapered outer and inner circumferential surfaces relative to one another, and integrally coupling the outer and inner tube members to each other.