In a single screw compressor, gates (51) of gate rotors (50) are to be engaged with spiral grooves (41) of a screw rotor (40). In each spiral groove (41) of the screw rotor (40), an area extending from a start point of the spiral groove (41) to a position in a compression stroke is a suction-side portion (45), and the remaining portion (portion up to a terminal point of the spiral groove (41)) is a discharge-side portion (46). In the discharge-side portion (46), a clearance between a wall surface (42, 43, 44) therein and the gate (51) is substantially “0 (zero).” A clearance between the wall surface (42, 43, 44) in the suction-side portion (45) and the gate (51) is wider than that between the wall surface (42, 43, 44) in the discharge-side portion (46) and the gate (51), and is gradually narrowed from the start point toward the terminal point in the spiral groove (41).