To prevent not only oil leak to the inside of a rotor chamber, but oil leak to the outside of a compressor.

In an oil-free screw compressor provided with first shaft seal parts 13, 14 communicated from the side of a rotor chamber 6 through first circular grooves 15, 16 to first open air communication holes, and second shaft seal parts provided with labyrinths on inner surfaces, and communicated through second circular grooves to second open air communication holes 21, 22, an oil extracting passage 35 to communicate the second circular grooves with an oil sump part 34 at a lower part inside a gear box 7 in which a speed increasing gear line to transmit rotation of a motor 8 to either one of a pair of a male and a female screw rotors rotatably contained in the rotor chamber 6 is formed.