1. An image segmentation method for segmenting an object, which is not present in a reference image, from an input image including the object using the reference image, comprising: the edge intensity distribution extraction step of extracting edge intensity distributions in the reference image and input image; the direction-classified line detection step of detecting line components in terms of predetermined directions for edges in the reference image and input image on the basis of the extracted edge intensity distributions; and the image region specifying step of specifying an existence range of the object in the input image on the basis of a distribution of differences between the detected line components in units of directions between the reference image and input image.