1. An image registration method comprising: obtaining a first reference image by photographing a reference target from a first viewpoint; obtaining a second reference image by photographing the reference target from a second viewpoint; obtaining reference coordinate-difference values that indicate difference in coordinates of corresponding pixels between the first reference image and the second reference image; obtaining parallax registration-error values based on a distance of a photographing target from the first viewpoint and the second viewpoint; obtaining a registration result of a first image of the photographing target taken from the first viewpoint and the second image of the photographing target taken from the second viewpoint based on the reference coordinate-difference values; and correcting the registration result based on the parallax registration-error values.