There is provided a retarder control device which is simple in constitution and can reliably apply braking. An engine rotational speed detector detects an engine rotational speed, gear step detection means detects a gear step of a transmission, an accelerator operation detector detects the operated state of an accelerator, and a retarder controller previously stores automatic braking amount determination data representing the relation between the engine rotational speed and the automatic braking amount of a retarder corresponding to each gear step of the transmission and controls the retarder according to the automatic braking amount of the retarder calculated from the automatic braking amount determination data based on the detected engine rotational speed and gear step of the transmission when the accelerator is judged to be off from the detected operated state of the accelerator.