PURPOSE: To inhibit rise in the temperature of a braking section applying an engine brake with increased level of a brake cooling oil by increasing the revolution speed of the engine with a shift down when the brake oil temperature of a dump truck rises.

CONSTITUTION: A controller 5 receives signals from a wheel speed detector 2, a change step detector 3, and a brake cooling oil temperature detector 4 and outputs a signal to an electric-hydraulic converter 6 to control the wheel velocity according to the set value from a wheel velocity setter 1. The revolutions are memorized in the controller 5 according to the wheel velosity and the change steps and based on the revolutions, a central operation unit 53 provides a shift down signal to raise the engine revolutions when the oil temperature rises. With the rise of the oil temperature, the level of the braking oil increases thereby checking the rise in the cooling oil temperature.