1. An image matching method implemented on a computer for matching a first image and a second image, comprising: performing image conversion processing based on a distance from a single reference position in each of the first image and second image and an angle formed by a straight line passing through the reference position and a reference axis including the reference position and generating a first converted image and a second converted image in a two-dimensional space defined by the distance and the angle; performing matching processing of the first image against the second image based on results of correlation processing at a plurality of different relative locations in a first direction and a second direction orthogonal to the first direction in the first converted image and second converted image generated during image conversion; and judging whether or not image conversion processing is performed, before performing image conversion processing: an image conversion processing is carried out to the first and second images, on the basis of a distance from reference positions in the first and second images, and an angle between lines passing through the reference positions and reference axis including the reference positions; a generation is carried out for third and fourth conversion images in a two dimensional space defined by the distance and the angles of a lower resolution than that of the first and second converted images; and a judgment is carried out on the basis of a correlation processing result in a different plurality of relative positions in a first direction and a second direction perpendicular to the first direction on the third and fourth converted images.