A turbomachine including an annular combustion chamber, a centrifugal compressor, an annular diffuser enabling the flow of gas leaving the compressor to be diffused and directing this flow of gas towards the combustion chamber, and an outer casing surrounding the outside of the combustion chamber is disclosed. The diffuser includes an upstream portion oriented radially and presenting diffusion passages connected to the outlet from the compressor, an intermediate portion that is elbow-shaped, and a downstream portion having a series of circularly spaced-apart flow-straightening vanes. The flow stream in the downstream portion is delimited externally by the outer casing. The outer casing presents a shoulder against which the outer downstream edge of the intermediate portion comes into abutment.