maximum set temperature as well as on the size of the gap between the rotors after such expansion subsequently cold rotor profiles which will be exhibited by the rotors when the rotors having the above mentioned hot roller profiles are cooled down to the normal temperature are determined by computation then the rotors 1 and 2 are worked by machining such as grinding in conformity with the cold rotor profiles the rotor profile of the male rotor 2 may be one that is generated by the profile of the female rotor 1 when both rotors are assembled together in the rotor casing 6 and synchronously operated under a load as will be described thus the requirement for precision of rotor profile of the male rotor is not so strict as that of the known screw compressor the rotors 1 and 2 are so profiled that a gap of a size e g 10 to 20 u03bcm less than a predetermined gap size is formed between the rotors 1 and 2 when these rotors are mounted in the rotor housing 6 the machined female rotor 1 is then coated with a film 10 formed by electroless plating of nickel which enables the film to be formed with a high degree of uniformity in thickness and high level of hardness during the plating particles 11 of a material having an extremely high level of hardness such as silicon carbide or oxidized alumina are dispersed in the plating material so that these particles exist in the film 10 a coating film 12 which is made of a material softer than that of the coating film 10 on the female rotor 1 is formed on the machined male rotor 2 the coating film 12 is formed by.