An apparatus for inspecting a pattern to detect a small pattern defect has an illuminating light source, as illuminating optical system having a plurality of illuminating portions for switching an optical path of illuminating light flux to a surface of board constituting the inspected object from a plurality of directions different from each other, a detecting optical system having a variable magnification using an object lens for condensing reflected diffracted light from the illuminated board, a focusing optical system having a variable magnification capable of focusing an optical image by converged reflected diffracted light with a desired focusing magnification and an optical detector for detecting the optical image focused by the focusing optical system to convert it into an image signal, an A/D converter for converting the image signal into a digital image signal, and an image signal processor for processing the digital image signal to detect the defect.