PURPOSE: To properly release the brake power on the start on an ascent by detecting the load weight of a vehicle which starts on the ascent by a sensor and changing the timing for holding a brake power according to the detected output.

CONSTITUTION: A computer 15 detects the slope of a road surface by a slope sensor 16, and stepwise sets the stroke value which is optimum for releasing the brake power according to the slope of the road surface, and a clutch 21 is connected by a clutch actuator 22. When the stroke read by a stroke sensor 23 exceeds a set value, a brake control valve 13 is opened. The load weight of a vehicle is detected by a load weight sensor 34, and the computer 15 sets the optimum clutch stroke according to the load weight. Accordingly, independently of the magnitude of the load weight, the brake power can be released with the optimum timing, and the retreat of the vehicle on the start on an ascent or the brake drag can be eliminated.