1. A road shape estimating device for a vehicle, comprising: a position determination device for determining a position of the vehicle; a data obtaining processing unit which obtains, from a database in a data recording unit of a navigation device mounted in the vehicle, interpolation point data for a plurality of shape interpolation points which are set along a road, wherein the shape interpolation points and linear segments joining adjacent interpolation points represent a shape of the road; a radius calculation processing unit which calculates 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 which detects a corner in the predetermined section by extracting any shape interpolation point with a radius of curvature equal to or smaller than a first threshold which is set in advance based on the radius of curvature at each of the shape interpolation points and sets the detected corner in the database, the detected corner being represented by a plurality of the shape interpolation points and segments; and a corner dividing processing unit, which calculates an average value of radii of curvature at respective shape interpolation points in the detected corner, which sets a second threshold based on the calculated average value of radii of curvature, which divides, into a plurality of corners, the detected corner at a shape interpolation point having a radius of curvature equal to or larger than the second threshold in the detected corner, wherein each of the plurality of corners is represented by plural shape interpolation points and segments, and which estimates a road shape for a road section in front of the determined vehicle position based on the plurality of corners; and a control device which controls the vehicle in accordance with the estimated road shape of the road section in front of the vehicle.