A centrifugal fluid machine includes an impeller (4) that has a front shroud (41) arranged on one side in an axial direction, a rear shroud (42) arranged on the other side in the axial direction, and a plurality of blades (43) provided side by side in a circumferential direction between the front shroud and the rear shroud and is rotatably supported within a casing (2); a suction passage (2A) that allows a fluid to be sucked therethrough in the axial direction toward the impeller with the rotation of the impeller; a discharge passage (2B) that allows a fluid delivered under pressure by the impeller with the rotation of the impeller to be discharged in a direction intersecting the axial direction of the impeller; and a first flow path (5A) that communicates with the discharge passage, leads to the suction passage through a gap between the casing and the rear shroud, and an opening (5Aa) that opens toward the downstream side of the suction passage in a suction direction of a fluid. The opening area of the opening is set so that the ejection speed (Vs) of a fluid ejected from the opening to the suction passage is matched with the suction speed (V) of a fluid sucked into the suction passage.