To provide a fluid flow straightening device wherein a fluid flow straightening device can be fixed easily in the inlet of a flow path and a flow path structure can be simplified and fitting work can be made more efficient.

This fluid straightening device 11 assembled in a fluid flow path so as to straighten a flow of fluid flowing in the flow path comprises a fluid flow straightening device body 16 formed by stacking alternately a plurality of cylindrical spacer blocks 12 and a plurality of mesh-shaped flow straightening members 14 and 15 which have external shapes nearly equal to the external shape of the spacer block, and a retaining ring 17 having a retaining part 17a having the nearly equal external shape to the external shape of the flow straightening device body, protruding outward from the peripheral surface, bent from a part near the root so as to have an angle in a direction opposite to the direction of the insertion of the fluid flow straightening device body into the flow path, and integrally provided with the end surface of the fluid flow straightening device body on a side opposite to the direction of insertion into the flow path.