To provide a hydrodynamic retarder eliminating any necessity of installing a pump device separately, saving a space for installation, reducing the number of components assembled to a car body and assembling manhour, and reducing the cost.

The pump device 19 for circulating hydraulic fluid is constituted of a pump chamber 55 storing the hydraulic fluid in the circumference of a rotary shaft 11 inserted in a retarder case 5 adjacent to a hydraulic fluid inlet 5b in the retarder case 5, and a pump impeller 91 attached to the rotary shaft 11 facing to the pump chamber 55 and rotating in the pump chamber 55 along with the rotary shaft 11 so as to pump the hydraulic fluid in the pump chamber 55 to a hydraulic fluid outlet 5a side of the retarder case 5. The pump device is incorporated in the retarder case 5.