1. An image processing apparatus comprising: storage means for storing a plurality of electronic data as target comparison images; input means for inputting a reference comparison image obtained from a printed material; grayscale feature amount extraction means for extracting a grayscale feature amount of the reference comparison image; retrieval means for retrieving a target comparison image corresponding to the reference comparison image from said storage means by using the grayscale feature amount extracted by said grayscale feature amount extraction means; and processing means for extracting the image retrieved by said retrieval means from said storage means and processing the extracted image.