PURPOSE: To provide a rotor clearance adjustment method for preventing a reverse flow and increasing supercharging efficiency in a non-contact screw type compressor.

CONSTITUTION: A rotor is caused to rotate via a timing gear fitted to each rotor shaft, and clearance between rotor gear surfaces is adjusted with the timing gear, thereby providing a clearance adjustment method for a rotary pump. In this case, backlash between the gear surfaces of each rotor and backlash between timing gears are respectively measured, and a backlash area between the timing gears is deviated from a backlash center position between the rotor gear surfaces, thereby forming a narrow clearance area between male rotor and female rotor gear surfaces. This narrow area is effective to prevent a reverse flow. Also, supercharging efficiency can be improved by directing the narrow area for an increase in a supercharging amount.