Oil-free screw compressor having atmosphere open hole formed in casing and communication hole formed in shaft seal device

Provided is an oil-free screw compressor that minimizes the pressure loss at an annular space for connecting an open air hole and a communication hole and that prevents lubricating oil from flowing in during unloading operation. The oil-free screw compressor is provided with: a casing 12 that has a rotor chamber 150; a bearing 22 that supports the rotating shaft 21 of a screw rotor; a shaft sealing device 20 comprising an oil seal section 31 and an air seal section 60; an open air hole 24a that is formed in the shaft sealing device; at least one communication hole 31a that is formed in the rotating shaft; and an annular space 25 that connects the open air hole with at least one communication hole. The annular space is provided with an inner circumferential annular space 24g that is formed on the inner circumferential side of the casing. The oil-free screw compressor is configured so that the expression shown is satisfied when the opening cross-sectional area of the inner circumferential annular space is denoted by S1, the total opening cross-sectional area of the communication holes is denoted by S2, the opening cross-sectional area of an i-th communication hole among the communication holes is denoted by S2i, and the number of communication holes is denoted by n (n is a natural number of 1 or more).