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; calculating an orthogonal projection of the binary format image data in a direction orthogonal to a direction of the character line while shifting along the binary format image data in the direction of the character line; detecting positions at which orthogonal projection data obtained in the calculating an orthogonal projection are above a predetermined boundary judgment threshold as segmenting-positions for segmenting the characters that compose the character line; obtaining rectangles bounding the characters based on the segmenting-positions detected in the detecting positions; 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; wherein the extracting character features extracts character features in the rectangles and calculates similarity with respect to standard character features.