To provide a variable capacity centrifugal compressor with a simple structure while preventing abrupt contraction flow, abrupt extention flow, leakage in flow of a diffuser, and reducing low of flow.

A centrifugal compressor 1 is prepared by forming a part or all of diffuser walls 5, 6 of an elastic plate 7. Width of a diffuser flow passage can be continuously varied by elastic deformation of the elastic plate 7. The elastic plate 7 has a ring-like form coaxial with a compressor, and is elastically deformed by being pressed from a rear side by means of a pressurizing ring 14. The pressurizing ring 14 can be pressed adopting combination of a plurality of can members and lever members, a plurality of hydraulic pressure pistons, or feeding pistons, a lead screw and a motor. Pressing plates 18, 19 are superposed on a rear side of the elastic plate 7, for preventing deformation due to fluid pressure inside the diffuser.