A road surface slope estimating device includes a forward/backward sensor that outputs a detected forward/backward of a vehicle, a forward/backward estimator that calculates an estimated forward/backward of the vehicle, and a road surface slope estimating device that calculates an estimated road surface slope value using the detected forward/backward and the estimated forward/backward. The road surface slope estimating device implements a momentarily rough terrain determination step to determine whether the vehicle is traveling on momentarily rough terrain or continuously rough terrain. When it is determined that the vehicle is traveling on momentarily rough terrain, the road surface slope estimating device fixes the estimated road surface slope value to an estimated road surface slope value obtained immediately before the determination is made.