In a pattern inspection apparatus for comparing images of corresponding areas of two patterns, which are formed so as to be identical, so as to judge that a non-coincident part of the images is a defect, the influence of unevenness in brightness of patterns caused by a difference of thickness or the like is reduced, whereby highly sensitive pattern inspection is realized. In addition, high-speed pattern inspection can be carried out without changing the image comparison algorithm. For this purpose, the pattern inspection apparatus operates to perform comparison processing of images in parallel in plural areas. Further, the pattern inspection apparatus operates to convert gradation of an image signal among compared images using different plural processing units such that, even in the case in which a difference of brightness occurs in an identical pattern among images, a defect can be detected correctly.