1. An apparatus for judging road surface gradients comprising a speed detecting means for periodically detecting wheel speeds of four wheels of a vehicle, a vehicle body speed calculating means for obtaining a vehicle body speed of the vehicle, a slip rate calculating means for obtaining a slip rate, a means for calculating acceleration of the vehicle body for obtaining an acceleration of the vehicle body of the vehicle, a determining means for determining whether the acceleration of the vehicle body is within a specified range proximate to zero, and a gradient judging means for judging a gradient of a road surface on the basis of the determination of the determining means.