The present invention concerns a rotary fluid machine, such as a screw compressor, a centrifugal compressor or a pump, which is provided with a thrust bearing mounting unit which is capable of keeping the natural frequencies of bending vibration of rotors (1, 2) thereof within ranges of permissible values. To provide such a mounting unit (11), the axial rigidity thereof is made to be at least as high as that of the thrust bearings (6, 7), and its radial rigidity is made to be not more than half that thereof, When a radial load is applied to the thrust bearings (6, 7), the bearing mounting unit thus formed is deformed in substantially the radial direction alone, and the thrust bearings bear substantially no radial load.