The oil-free screw compressor includes: a screw rotor, a bearing configured to support a rotor shaft, a shaft seal device including an oil seal arranged at a position closer to the bearing and a gas seal arranged on at a position closer to the rotor chamber, an atmosphere communication space communicating with an atmosphere communication hole, and an oil return groove including a return inclined surface inclined so as to expand in diameter radially outward from a gas seal-side end to an oil seal-side end. The oil seal portion is a viscoseal including a spiral groove portion. An inner peripheral surface of the oil seal includes an inner peripheral boundary portion that defines a part of the atmosphere communication space, on a side closer to the bearing in the atmosphere communication hole. The oil seal-side end is arranged in a vicinity of the inner peripheral boundary portion and at a position closer to the bearing.