Abstract: In a multi-stage centrifugal compressor, having a plurality of impellers mounted on a rotation shaft, or a single-stage centrifugal compressor, a vaned diffuser, having a plurality of vanes, is mounted at a downstream side of the impeller, and a vaneless diffuser portion is formed downstream of this vaned diffuser. A meridian plane cross-sectional shape of each of two wall surfaces forming the vaneless diffuser portion provided downstream of the vaned diffuser is such that its channel height is decreased progressively in a downstream direction. Instead of this vaneless diffuser, there can be provided a second vaned diffuser having a small number of vanes. In this case, also, two wall surfaces, forming the second vaned diffuser, are contracted progressively in the downstream direction.