PURPOSE: To provide a braking force control method capable of realizing constant speed traveling by setting a braking force to an optimal value according to a movable load without forcing a driver to perform burdensome switching operations during traveling.

CONSTITUTION: A movable load detecting sensor 10 for detecting the movable load of a vehicle is provided and by means of a brake device control unit 8 for controlling the operations of auxiliary brake devices 3 and 4 for producing braking forces, the operations of the auxiliary brake devices 3 and 4 are controlled based on the detecting signal of the movable load detecting sensor 10 so as to obtain larger braking forces when the movable load is large.