To realize a natural decelerating feeling even if a car speed is varying by adjusting a rising speed of braking force according to driving conditions at a process to heighten this braking force gradually from a start output, and setting the rising speed at the time of a high speed to be smaller than that at the time of a low speed.

In an auxiliary braking control unit 32, if an idle switch 22 is turned on in a state that an auxiliary brake switch 29 is set to On, operations of a retarder 13 and an exhaust brake 19 are started, and those of inclination and braking force are gradually increased till reaching to the maximum braking force, and then a level of the maximum braking force is maintained. In addition, a rising inclination in the braking force is adjusted according to a car speed, and thereby at the time of a high speed, the rising speed is set to be smaller than that in time of a low speed. With this constitution, even if the car speed varies, a natural, effective decelerating feeling is securable likewise.