To estimate a vehicular running state from an operation information and carry out the shift down to the gear step which can increase the auxiliary brake effect after speed change to the maximum in response to the running state at the operation time of the auxiliary brake for increasing the negative work of an engine.

This is provided with a means (step 3-9) judging a vehicular running state from the time series information of a car speed, a means (step 10-14) for finding out the gear step started when at least the operation condition of the auxiliary brake is established at the judging time of the running state wish to apply the auxiliary brake effectively and which can increase the auxiliary brake effect after the speed change to the maximum in the engine allowable rotation from the detection information of the car speed and the shift position information of a transmission and for controlling the shift down to the gear step, a means (step 15,18) for counting the number of times of the shift down control and a means (step 12-17) correcting so as to increase the shift up point of a car speed map in response to the number of times of the shift down control and an accelerator opening.