1. A visual inspection method comprising: taking a photograph of an illuminated inspection target sample from above to generate an original picture; using a processor to perform steps comprising: converting said original picture obtained by said taking into a binary picture; labeling said binary picture obtained by said converting to generate a labeling picture; calculating a summation of shade values of said original picture corresponding to said labeling picture generated by said labeling; judging a pass or rejection of said inspection target sample based on said summation of the shade values obtained by said calculating; calculating an area of said labeling picture; calculating an average shade value of said original picture corresponding to said labeling picture by dividing said summation or said shade values of said original picture by said calculated area; and calculating a total average shade value by averaging all or said calculated average shade values, wherein said judging is performed by judging said pass or rejection of said inspection target sample based on said calculated total average shade value.