The present invention provides a defect inspection system which enables an improvement in the efficiency of spatial filter settings, and at the same time enables automation of the spatial filter settings. An adjustable field-of-view diaphragm is narrowed to obtain an image of a spatial filter surface by use of an observation camera, and pixels of the image are classified into a plurality of groups according to the brightness level of bright spots of diffracted light. A spatial filter is set in such a manner that a group, the brightness level of which is highest, is light-shielded, and an observation image is then captured. Whether or not a repetitive pattern remains in the captured image is determined, and when it is determined that a repetitive pattern remains, the settings of the spatial filter are changed. The spatial filter is set in such a manner that in addition to the group which has been light-shielded earlier, a group, the brightness level of which is the highest next to the light-shielded group, can also be light-shielded. The same step is repeated until it is determined that no repetitive pattern remains. The settings of the spatial filter then end.