1. An image processing method for stitching a plurality of images including regions which are to be overlapped with each other at said regions to form a single image, comprising steps of accepting input of a first image, a second image including a region to be overlapped with said first image, and a third image including a region to be overlapped with said second image; matching scales of said inputted images to each other; obtaining a plurality of corresponding points corresponding between said first image and said second image which match each other in scale; obtaining a plurality of corresponding points corresponding between said second image and said third image which match each other in scale; calculating a planar projective transform matrix indicating geometric relationship among said first through said third images based on the obtained corresponding points between respective images; executing planar projective transformation of said second image based on the calculated planar projective transform matrix; and stitching said second image after the planar projective transformation with said first and said third images.