1. An image processing apparatus, that processes an original image containing a plurality of pixels obtained by sight-through of a subject, comprising: an evaluation image producing means operative to produce an evaluation image that evaluates whether each pixel is a linear structural object in said original image based on said original image; a direction image producing means operative to produce a direction image that indicates the direction to which a line extends, given each pixel is the linear structural object in said original image based on said original image; a difference image producing means operative to produce a difference image in which a pixel value other than said linear structural object is subtracted from the pixel value of the portion of said linear structural object based on said direction image; and an extraction image producing means operative to produce the extraction image in which the linear structural object in the original image is extracted based on said evaluation image and said difference image.