PURPOSE: To obtain an oilless screw compressor of which the gap between male and female rotors can be always maintained in a minute and constant condition by connecting a temperature regulating valve and a cooler in order to the jacket provided in a casing, and communicating the temperature regulating valve to the inlet side of the cooler.

CONSTITUTION: A compressor body 1 is provided with a casing 5 and a pair of male and female rotors 6, 7 mutually engaged. The casing 5 is provided with a suction port 2, a discharge port 3, and a jacket 4. On the other hand, a refrigerant circulating route of the jacket 4 is constituted out of a jacket inlet port 10, a temperature regulating valve 11, a cooler 12, a pump 13, and a jacket outlet port. The temperature regulating valve 11 is communicated to the inlet side of the cooler 12. As the inlet side of the cooler 12 and the temperature regulating valve 11 are by-passed, temperature of the refrigerant flowing in the jacket 4 can be controlled by adjusting flow of the refrigerant not to pass through the cooler 12 with the temperature regulating valve 11. Thus, the discharge temperature can be held constant, and the gap between the male and female rotors 6, 7 can be held in the minute and constant condition.