To automatically detect traveling on a long downward slope to make effective use of an auxiliary brake system.

A positive flag is erected by a switch of an auxiliary braking device based on driving operation, and a negative flag is erected by the operation of an accelerator pedal. The flag value is repeatedly read in a short cycle and cumulatively counted by a counter S, and when the count value exceeds the specified value, a downward slope detection signal D is sent out. Traveling on a downward slope is automatically judged from the state of driving operation without judging it by measuring the physical quantity such as the output of a gradient sensor. Traveling on the long downward slope is automatically judged, and an auxiliary brake system is actuated to attain the effective regeneration of braking energy.