A screw compressor in which a casing is prevented from being brought into contact with a gate rotor while gas is prevented from leaking through the clearance between the casing and the gate rotor. With respect to the width of the seal surface (11) of the casing, the width (Wd) on the gas discharge side of a screw rotor (2) is larger than the width (Ws) on the gas suction side of the screw rotor (2).
