PURPOSE: To adapt the braking force to the gradient of the slope and changes in the load weight by controlling the braking force according to the deviation in the response between the actual wheel velocity and the reference model by means of the model adapted to respond to the input of velocity command provided in a controller of a dump truck.

CONSTITUTION: A controller 3 has a reference model 4 comprising a microcomputer, a gain adjustor 5 and an amplifier 6 to control a retarder braking of a dump truck. When a command wheel velocity signal corresponding to a command wheel velocity Vc and an actual wheel velocity signal eV is inputted into the reference model 4, it sequentially outputs an ideal wheel velocity signal Em corresponding to an ideal wheel velocity based on a deviation Δe from the command wheel velocity Vc. The gain adjustor 5 outputs a signal es according to a deviation ΔE between the actual wheel velocity signal Ev and the ideal wheel velocity signal Em to the amplifier 6 to adjust a gain K of the amplifier 6. On the other hand, the amplifier 6 amplifies the input signal Δe with an amplifier 6 and applies it to an electric-hydraulic converter 8 as a control signal KΔe. The sum of an output Fs from the converter 8 and a disturbing force F is applied to the dump truck 2.