1. An image processing apparatus which executes image processing of determining a comparison region used to perform similarity comparison between a comparison source image and a comparison destination image, comprising: histogram calculation means for calculating horizontal and vertical projection histograms of an image to be processed; object size calculation means for calculating autocorrelation values of the horizontal and vertical projection histograms which are calculated by said histogram calculation means, and calculating horizontal and vertical object sizes until the autocorrelation values decrease to predetermined thresholds; aspect ratio estimate value calculation means for calculating an aspect ratio estimate value of a circumscribed rectangle of an object serving as a comparison region candidate in the image to be processed, on the basis of calculation results of said object size calculation means; and comparison region determination means for correcting the comparison region candidate on the basis of the aspect ratio estimate value calculated by said aspect ratio estimate value calculation means, and determining a final comparison region.