Abstract: A rotor disposed in a fluid retarder which includes a annular shell having a semicircular cross section and a plurality of blades, each blade being composed of a semicircular blade body and a rib bent from an arcuate edge of the blade body and fixed to an inside of the shell. The annular shell and the blades are composed of sheet metal. The blades are arranged at regular intervals in the circumferential direction of the shell and are welded into place. Because the annular shell and the blades are composed of sheet metal and are brazed together, the cost of manufacturing the rotor can be reduced without reducing its performance.