1. A road shape estimating device, comprising: a data obtaining processing unit for obtaining interpolation point data for a plurality of shape interpolation points which are set along a road and represent a shape of the road; a radius calculation processing unit for calculating a radius of curvature at each of the shape interpolation points based on the interpolation point data for a predetermined section of the road; a corner detection processing unit for detecting a corner in the predetermined section based on the radius of curvature at each of the shape interpolation points; and a corner dividing processing unit for setting a threshold corresponding to an average value of radii of curvature at respective shape interpolation points in the detected corner and for dividing the corner at a shape interpolation point having a radius of curvature equal to or larger than the threshold in the detected corner.