To provide a repairing method for an oilless screw compressor capable of saving repairing costs as well as effectively utilizing a utilizable member as much as possible.

After an inner surface is concentrically cut out around rotor axial cores OM, OF until a corrosion part 5 is eliminated in an inner surface of a rotor chamber 2 for housing a pair of male and female screw rotors 3, 4 which are engaged with each other, screw rotors 3, 4 expressed with new two-dot-chain lines are employed instead of the screw rotors 3, 4 expressed with full lines. In the screw rotors 3, 4, rotor diameters increased for cut-out amount of the inner surface are formed in the rotor chamber 2, and increasing ratio of one gear groove volume of the screw rotors 3, 4 against an amount before cut-out it, not attain an increasing ratio of an allowable output upper limit against a nominal output of a rotor driving motor.