1. A negotiable instrument processing apparatus comprising: an image reading unit that scans a negotiable instrument and outputs an image; a threshold value determination unit that sets a threshold value used for digitizing an image, the threshold value based on a density level frequency distribution of gray scale image data being obtained from a first scanning area of the negotiable instrument when the image data reading unit scans the first scanning area, the first scanning area containing part of a printed text area where text is printed on the negotiable instrument and part of a background of the negotiable instrument; and a digital conversion processing unit that digitizes and converts at least one image to digital image data based on the threshold value set by the threshold value determination unit, the at least one image being obtained from a second scanning area of the negotiable instrument when the image reading unit scans the second scanning area.