Background images are removed and only desired payment information is extracted from a grayscale image of a check or similar financial instrument. A density distribution improvement process applied to grayscale raw image data 40 acquired by scanning a check corrects the density distribution of the raw image data 40 to separate the density range of the desired payment information from the density range of the background image. A threshold level 44 for clearly separating the payment information from the background image is then calculated from the characteristics of the density distribution of the grayscale improved image data 42 acquired by the density distribution improvement process. Histograms of the density distributions are used to determine the characteristics of the density distribution. The improved image data 42 is then converted to binary image data 46 using the threshold level 44.