A processor executes a program and reduces reference and comparative images to reduced reference and comparative images, in a ratio of ½, by setting a pixel signal value of each pixel in each image using pixel signal values for each of four pixels in each of the images. The processor specifies (i) a first corresponding point similar to a first feature point included in the reduced reference image, from the reduced comparative image, using a first filter for 3×3 pixels, and (ii) a second corresponding point similar to a second feature point in the reference image, corresponding to the first feature point, from the comparative image, using a second filter for 5×5 pixels, on the basis of a position of the first corresponding point specified in the reduced comparative image.