In a constant speed drive (in an auto cruise), the procedure of the invention executes proportional control to set an accelerator opening Acc corresponding to an observed vehicle speed and a target vehicle speed. When the accelerator opening Acc is less than a preset opening A2 included in a dead zone where a torque demand Tr* is kept equal to 0, the procedure corrects the accelerator opening Acc to have a corresponding torque demand Tr* included in a negative zone where the torque demand Tr* takes negative values and linearly increases. This arrangement heightens the linearity of the graph of the torque demand Tr* against the accelerator opening Acc in the constant speed drive, compared with the linearity in a standard drive.