1. An optimal image-quality selection apparatus comprising: means for quantizing a signal representing an image, obtained from a scanner, to output two or more video signals representing the image by different depth levels; means for detecting and counting all the graphic blocks included in each of the two or more video signals, the graphic block being defined as a chain of dots in the image; means for detecting and counting the number of graphic blocks having a predetermined size in each of the two or more video signals; means for calculating a ratio of the number of graphic blocks having the predetermined size to the number of the all graphic blocks included in each of the two or more video signals, respectively; and means for selecting a largest one out of the ratios corresponding to the two or more video signals to output a signal representing that a depth level corresponding to the largest ratio is optimal.