To improve sensitivity of a defect inspection, it is required to decrease influence of excessive diffraction from a spatial filter. Further, it is preferable to secure signal intensity from defects and particles as much as possible, while the influence of the excessive diffraction is decreased as much as possible. The present invention is characterized in setting a width of a spatial filter surface such that an unnecessary image caused by diffraction, that is, an intensity of the excessive diffraction is sufficiently small with respect to an intensity of a desired image. In the present invention, an SN ratio that is an index for deciding a width of the spatial filter is calculated from a region subjected to the influence of the excessive diffraction in an inspection image, and a width of a shield unit of the spatial filter is set so as to maximize the SN ratio.