1. A method for generating an orthophoto image comprising the steps of: (A) divisionally-capturing a target region from a platform; and (B) thereafter generating an orthophoto image of an entire target region by applying an orthogonal projection transformation processing to respective divisionally-captured images by use of altitude information of an elevation model of the target region, wherein in the step of (A) divisionally-capturing the target region, a plurality of times of image capturing are repeated along a flight direction of the platform flying above the target region with a plurality of cameras as area sensors mounted in a direction perpendicular to the flight direction of the platform flying above the target region by (a1) rendering viewing angles of the plurality of cameras as being different from each other as well as rendering focal lengths of the plurality of cameras as being different from each other while maintaining a ground resolution of each of the plurality of cameras at a constant reference that is a predetermined ground resolution, along a direction perpendicular to the flight direction, and by (a2) maintaining the viewing angles of each of the plurality of cameras at the same in the flight direction as well as maintaining the focal lengths of each of the plurality of cameras at the same, along the flight direction, and wherein in the step of (B) generating the orthophoto image of the entire target region, (B1) a plurality of elevation models of strip-shaped regions are generated by stereo-matching a plurality of divisionally-captured images, which have been obtained in the same viewing angle and the same focal length by each said camera of said plurality of cameras, in the flight direction by use of tie points thereon with respect to image capturing overlap regions, image capturing position information of each camera of said plurality of cameras, and image capturing orientation information of each camera of said plurality of cameras, where the tie points have a function of associating relative positions of the divisionally-captured images to be matched in the flight direction, (B2) thereafter the plurality of elevation models of strip-shaped regions are matched in a direction perpendicular to the flight direction among overlap regions corresponding to image capturing overlap regions in the plurality of times of capturing performed in such a manner that the plurality of elevation models adjacent to each other are adjusted with respect to coordinate values of cells thereon corresponding to each other in the overlap region therebetween, and thereby the elevation model of the target region is generated, whereby the cells belonging to said overlap region are adjusted to each other such that the coordinate values of the adjusted cells satisfy a linear expression for adjustment; and (B3) whereby accuracy of each of the plurality of elevation models of the strip-shaped regions is enhanced while decrease of accuracy of the elevation model of the target region is suppressed.