1. An appearance inspection method, comprising: (a) providing an image data in which an inspected sample is photographed; (b) detecting a brightness of each of a plurality of image units included in said image data based on said image data; (c) detecting the number of said image units being identical with each other in said brightness for each of said brightness; (d) detecting, as a measured maximum number, the number that is maximum of the detected numbers as a result of said (c); (e) computing said measured maximum number to determine a set maximum number; (f) determining a threshold level of said brightness based on said set maximum number; (g) converting said image data into a binary pattern based on said threshold level; and (h) detecting a defect of said inspected sample based on said binary pattern.