1. An image processing apparatus comprising: a feature point extracting section for extracting, from first and second document image data among plural inputted document image data, first feature points of the first document image data and second feature points of the second document image data, a features calculating section for calculating first features of the first feature points and second features of the second feature points, a matching process section for comparing the first features with the second features, and for extracting a plurality of first feature points and a plurality of second feature points whose features agree with each other, a coordinate transformation section for transforming a coordinate system of the second document image data into a coordinate system of the first document image data in accordance with coordinates of the first and second feature points extracted by the matching process section, and a logical AND operation section for performing a logical AND operation of the first document image data and the second document image data which has been processed by the coordinate transformation section so as to extract image data, having a common format, contained in the first and second document image data.