1. An apparatus for inspecting pattern on an object, comprising: an image pickup device for performing an image pickup of an object to acquire data of gray-scale inspection image; a memory for storing data of reference image; a region class specifying circuit for specifying a region class to which each pixel of said inspection image belongs on the basis of a pixel value of said reference image; and a check circuit for checking each pixel of said inspection image by using a parameter value in accordance with a corresponding region class.