Abstract: One of male and female rotor elements is coated with a coating which is different in hardness from the other uncoated element. Even when the rotor elements come to contact with each other during the operation due to thermal expansion or the like, because of the difference in hardness between the coating and the other uncoated element, either of them which has a less degree of hardness is scraped off. As a result, accident due to contact between the male and female rotor elements can be prevented and the screw rotor can be operated with an optimum clearance between the elements.