A method and apparatus for detecting defects are provided for detecting defects or foreign matter on an object to be inspected. The apparatus includes a movable stage for mounting a specimen, an illumination system for irradiating a circuit pattern with light from an inclined direction, and an image-forming optical system for forming an image of an irradiated detection area on a detector from the upward and oblique directions. With this arrangement, diffracted light and scattered light caused on the circuit pattern through the illumination by the illumination system is collected. A spatial filter is provided on a Fourier transform surface for blocking the diffracted light from a linear part of the circuit pattern. The scattered and reflected light received by the detector is converted into an electrical signal. The converted electrical signal of one chip is compared with that of the other adjacent chip.