An apparatus and a method for processing checks and other negotiable instruments efficiently captures MICR data required for electronic payment and an image of the check or negotiable instrument enabling the amount, payee, and other information printed or written on the check face to be clearly read. The image reading unit 11 of the check processing apparatus 110 scans and outputs a gray scale image of the check. A threshold value determination unit 12 sets a threshold value for digitizing the image data based on the density level frequency distribution of primary gray scale image data obtained by the image data reading unit scanning a first scanning area T. This first scanning area contains part of a printed text area 127 where text is printed on the negotiable instrument and part of the background 49 of the negotiable instrument. A digital image processor 20 digitizes and converts secondary gray scale image data to digital image data based on the threshold value set by the threshold value determination unit 12. The secondary gray scale image data is generated by the image reading unit 11 scanning a predefined second scanning area of the negotiable instrument.