1. An image processing apparatus comprising: a density distribution improvement means for receiving grayscale raw image data containing both first image information and second image information, said raw image data having a density distribution spanning from a low density side defining a first density distribution range to a high density side defining a second density distribution range, said high density side being higher than said low density side, modifying the density distribution of the raw image data to shift the first image substantially to the low density side and to shift the second image substantially to the high density side, 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.