1. An image processing apparatus comprising: a density distribution improvement means for receiving grayscale raw image data containing both background image information and foreground image information, modifying a density distribution of the raw image data to increase a difference between a density distribution range of the background image information and a density distribution range of the foreground image information, and generating grayscale improved image data; a threshold level calculation means for receiving the improved image data and calculating a threshold level for two-level digital conversion based on a density distribution in the improved image data; and a digitizing means for receiving the improved image data and the threshold level, digitizing the improved image data using the threshold level, and outputting two-level image data.