Abstract: Improvement to supersonic centrifugal compressors comprising a set of radial blades fixed to a mobile circular rim chocked on the compressor shaft and extending beyond the blades by a flat ring limiting, with a fixed upper housing, a semi-fixed, semi-mobile diffuser and in which the distance separating the axis of rotation of the machine from a trailing edge of the mobile blades is greater in the vicinity of the fixed housing than in the proximity of the mobile rim; the invention enables a good distribution of the flow speed of the fluid to be obtained at the output of mobile blades in the semi-fixed semi-mobile diffuser.