A screw compressor (10) is provided with shaft seal sections (28, 41) located on opposite sides of a compression chamber (24) of a screw rotor (25), to prevent mixing of oil in bearings (27, 39, 40) into the compression chamber (24) and leakage of process gas from the compression chamber (24). A suction-opening return line (52) interconnects the shaft seal section (41) on the discharge side of the compression chamber (24) and a suction opening (17) of the compressor body (11). A supply-process-gas communicating line (61) interconnects the suction opening (17) of the compressor body (11) and the upper part of the oil supply tank (13), and a shaft seal section (53) divides the inside of the compressor body (11) and an atmospheric environment.