1. An object monitoring method for monitoring the presence or absence of a change in an object by performing a comparison between a reference image acquired by capturing the object and a comparison image acquired by capturing the object, the object monitoring method comprising: a pixel value correction step that causes an average and variance of image information values of pixels (each of which will be referred to as a “pixel value,” hereinbelow) in a second subregion inclusive of a first subregion inclusive of being formed of only one pixel in a comparison image resolved into a plurality of 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 a plurality of pixels, thereby to execute a calculating operation that corrects the pixel value of each pixel in the first subregion in the comparison image by recognizing each of a plurality of subregions to be the first subregion, the plurality of subregions being obtained when the comparison image is divided into the plurality of subregions respectively inclusive of being formed of only one pixel; and a change determination step that determines the presence or absence of a change in the object by performing a comparison between the reference image and a post-correction comparison image acquired through the pixel value correction step.