1. A gradient estimation apparatus for a host vehicle comprising: a feature point extracting unit configured to extract feature points based on two images captured in a predetermined time interval by an imaging unit which is incorporated in the host vehicle; an object detecting unit configured to detect more than two image regions indicating distinct moving objects from the two images captured by the imaging unit; and a gradient calculating unit configured to calculate a gradient of a road surface on which the moving objects are located, based on coordinate values of the feature points extracted by the feature point extracting unit in the image regions and amounts of movement of the coordinate values of the feature points over the predetermined time interval, wherein the imaging unit includes a lens and an imaging surface, and converts the feature points into a normalized camera coordinate system on the imaging unit, where a distance from the lens and the imaging surface is normalized as 1, the gradient calculating unit calculates a gradient “α” using Equation (3), defined in the normalized camera coordinate system, based on more than two selected sets of two feature points which are selected differently from the more than two image regions and which have coordinate values Yk and Yj located along a vertical direction, and the amounts of movement ΔYk and ΔYj in the predetermined time interval in the vertical direction of the feature points k and j of the selected set, where k and j are integers that are equal to or greater than 1 and smaller than N obj , N obj represents the number of the selected feature points, α = ∑ k > j ⁢ ⁢ Yk Δ ⁢ ⁢ Yk - Yj Δ ⁢ ⁢ Yj / ∑ k > j ⁢ ⁢ 1 Δ ⁢ ⁢ Yk - 1 Δ ⁢ ⁢ Yj Equation ⁢ ⁢ ( 3 ) and in Equation (3), Σ(k>j) represents the sum in the sets of the feature points in which k>j among the sets of the feature points k and j.