To provide a rotor for a fluid machine without generating deflection in an axial direction or in a rotating direction to a shaft even when rattling is generated between the rotor and the shaft due to thermal expansion difference.

At installation parts of the shafts 20 and 21 for the rotors 22 and 23, a left-handed thread part 30 and a right-handed thread part 31 are formed in parallel to the axial direction. The rotors 22 and 23 are molded by casting integrally with the shafts 20 and 21. When rattling is generated between the shafts 20 and 21 and the rotors 22 and 23, the rotors 22 and 23 are respectively moved in the axial direction and rotated in opposite directions to each other by the left-handed and the right-handed thread parts 30 and 31. Position deflection due to rattling is thus prevented.