PURPOSE: To safely perform retarder control at intense cold site and as well as eliminate working lag by a method wherein a retarder brake is put in operation by means of hydraulic pressure converted by electric signal issued when the actual vehicular speed exceeds the command vehicular speed.

CONSTITUTION: A vehicular speed setter 2 outputs the vehicular speed command signal in accordance with the position of a vehicular speed control lever 1 to the controlling device 30. Similarly, a temperature detector 5 outputs the signal correspond to the oil temperature of the retarder brake and a vehicular speed detector 6 outputs the signal proportional to the actual vehicular speed to the controlling device 30. When the controlling device 30 judges that the actual vehicular speed exceeds the command vehicular speed, the controlling signal Sa is outputted from the device 30 to actuate converters 10 and 11 so as to apply the retarder brake 12. Furthermore, when the vehicular speed does not change even under the condition that said brake 12 is put in operation and the oil temperature of the brake exceeds the set allowable temperature, the controlling signal Sb is outputted from the controlling device 30 to actuate converters 22 and 11 so as to apply the retarder brake 12 with the braking effort weaker than usual and as well as to prevent the oil temperature from rising.