To make it possible to determine a flat road with a high degree of accuracy without being influenced by the detection error of an acceleration sensor from the detected value in the acceleration sensor which detects the slope of the road face when performing the neutral control of an automatic transmission on the flat road.

After turning on an ignition switch, the compensation condition of the acceleration sensor 26 is determined (S1-S4). In the case where the conditions are met (S6), the compensation experience flag Fg is set (S7) while computing the acceleration study compensation value by subtracting the calculation acceleration Go which is computed from the amount of change per unit time of the car speed Vsp detected by the speed sensor 24 from the acceleration Gx detected by the acceleration sensor 26. The value of the compensation experience flag Fg is taken in when performing the neutral control of the automatic transmission 2, the neutral control is performed only in the case of Fg=1 and the flatness of a road is determined from the value which the acceleration Gx is corrected by the acceleration study compensation value .