1. A method of inspecting a substrate for a defect, comprising the steps of: obtaining a difference image between an image of a normal substrate and an image of an inspection object; rotating the difference image 360 degrees by every predetermined angle about a center of the substrate, and synthesizing the difference image before the rotation and the difference image after the rotation so that each pixel value of the image after the synthesis is increased; digitizing using a Zernike polynomial the image obtained by the synthesis; and determining the presence or absence of a defect on the substrate based on a Zernike value of a concentric circle component out of Zernike values digitized by the Zernike polynomial.