1. A character line recognition method for processing image data obtained by scanning a character line on a medium to recognize the character line, the method comprising: processing the image data into monochrome binary format image data by using a predetermined binarization standard threshold; extracting character features from each character that composes the character line to calculate similarity with respect to standard character features; temporarily determining characters based on the similarity; calculating basic statistics of the similarity for all the characters which have been temporarily determined; and changing the binarization standard threshold based on the basic statistics and then returning to the processing the image data.