1. A method for restoring a scan image from an image system using X-rays, the method comprising the steps of: obtaining a slit image through a slit inclined at a predetermined angle; measuring a blurring function oversampled with respect to the slit image; obtaining the scan image of a subject; and restoring the scan image to increase a resolution of an interface of the subject using the measured blurring function, wherein the step of measuring the blurring function includes the sub-steps of: generating line spread functions (LSFs) with respect to a plurality of alignments sampled with respect to the slit image; and measuring a modulation transfer function (MTF) as the blurring function of the slit image using the plurality of LSFs.