A centrifugal compressor for use in turbo-refrigerators, for example, has a motor casing frame section in which a drive motor is accomodated and a gear casing frame section in which a multiplying gear mechanism is accomodated. The multiplying gear mechanism is connected at its input side with the drive motor and at its output side with an impeller. The mechanism has a first shaft including a rotor shaft of the drive motor and a gear shaft carrying a multiplying gear and separably connected directly with the rotor shaft, a second shaft mounting the impeller and having a pinion meshing with the speed multiplying gear, and bearings mounted on bearing casing frame sections for supporting the first and second shafts at their opposed ends, respectively. The motor casing frame section, gear casing frame section and bearing casing frame section are integrally formed as a unitary body.