An automatic retarder controller for a vehicle, which can prevent overheating, and can more precisely control the vehicle speed to remain constant. For this purpose, in the automatic retarder controller which is mounted on a load-carrying vehicle together with a cooled retarder (31) for exerting a braking force in response to a driving signal, and which automatically controls the driving signal so that the slope descending speed of the vehicle remains nearly constant, a detector for detecting the loading weight of the vehicle is included, and the controller impresses the gain corresponding to the detected loading weight upon the driving signal while the controller controls the vehicle speed to remain constant.