1. An image processing system for carrying out a pattern matching between a first image and a second image, comprising: a gravity point distribution image generator for generating a gravity point distribution image based on the first image, pixel values of the gravity point distribution image respectively varying with distance from a gravity point which is a center of a pattern in the gravity point distribution image; a contour line image generator for generating a contour line image consisting of a plurality of contour line patterns based on the gravity point distribution image; an edge line segment group generator for generating a group of edge line segments based on the second image; and a pattern matching unit for carrying out a pattern matching between the plurality of contour line patterns and an edge line segment constituting the group of edge line segments.