1. A diagnostic apparatus for detecting abnormalities in an image recognition system including an image input unit and an image recognition unit for receiving as input an image signal from said image input unit and performing image recognition on said image, provided with a histogram computing unit to perform predetermined computations on an image signal fetched into said image recognition unit to generate a density histogram and an abnormality judging unit to analyze the density histogram generated by said histogram computing unit to judge the presence of an abnormality.