A centrifugal fluid machine comprising: a casing having a hollow shape; a disk member having a boss portion fixed to a rotary shaft, the boss portion including a front boss portion; an impeller fixed to the rotary shaft that is rotatably supported within the casing and has an annular member arranged on one side in an axial direction, the disk member is arranged on the other side in the axial direction, and a plurality of blades provided side by side in a circumferential direction between the annular member and the disk member; a suction passage that allows a fluid to be sucked therethrough in the axial direction from the center of the annular member in the impeller with the rotation of the impeller; a discharge passage 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 flow path that communicates with the discharge passage and leads to the suction passage through a gap between the casing and the disk member, and has an opening that opens toward the downstream side of the suction passage, characterized in that the opening opens in a suction direction of a fluid, and wherein 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 set to be in a range of 0.5V to 1.5V, where V denotes a suction speed of a fluid sucked into the suction passage, the flow path includes a through hole which is provided through the boss portion along the axial direction, and the through hole includes a front through hole in a form of an annular groove which is formed at the front boss portion and is continuous in the circumferential direction of the front boss portion. a casing having a hollow shape; a disk member having a boss portion fixed to a rotary shaft, the boss portion including a front boss portion; an impeller fixed to the rotary shaft that is rotatably supported within the casing and has an annular member arranged on one side in an axial direction, the disk member is arranged on the other side in the axial direction, and a plurality of blades provided side by side in a circumferential direction between the annular member and the disk member; a suction passage that allows a fluid to be sucked therethrough in the axial direction from the center of the annular member in the impeller with the rotation of the impeller; a discharge passage 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 flow path that communicates with the discharge passage and leads to the suction passage through a gap between the casing and the disk member, and has an opening that opens toward the downstream side of the suction passage, characterized in that the opening opens in a suction direction of a fluid, and wherein 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 set to be in a range of 0.5V to 1.5V, where V denotes a suction speed of a fluid sucked into the suction passage, the flow path includes a through hole which is provided through the boss portion along the axial direction, and the through hole includes a front through hole in a form of an annular groove which is formed at the front boss portion and is continuous in the circumferential direction of the front boss portion.