A method includes a pixel-value correction step that causes an average and variance of pixel values in a second subregion inclusive of a first subregion inclusive of being formed of only one pixel in a comparison image resolved into pixels to match with an average and variance of pixel values in a subregion corresponding to the second subregion on a reference image similarly resolved into pixels, thereby to execute calculation that corrects the pixel value of each pixel in the first subregion in the comparison image by recognizing each of subregions to be the first subregion, the subregions being obtained when the comparison image is divided into the subregions respectively inclusive of being formed of only one pixel; and a change determination step that determines the presence/absence of a change in the object by comparing the reference image with a post-correction comparison image acquired through the pixel value correction step.