To provide a speed reduction controlling device for vehicle which can suppress a sense of incongruity occurring at the time of the lost of a preceding vehicle when a speed reduction is generated by controlling a change gear and a braking device while making them cooperate in order to correct a positional relationship with the preceding vehicle.

This speed reduction controlling device for vehicle generates a speed reduction by controlling the change gear 10 and the braking device 200 while making them cooperate in order to correct the positional relationship between a vehicle X1 and the preceding vehicle X2 which is ahead of the vehicle. When the presence of a possibility for missing the preceding vehicle is judged, the ratio of the speed reduction generated by the change gear is increased. The presence/absence of the possibility for missing the preceding vehicle can be judged from a distance Lb' between the vehicle and the preceding vehicle, and the curvature or the radius R of a curve ahead of the vehicle. Also, the presence/absence of the possibility for missing the preceding vehicle can be judged from a road slope.