1. An image processing apparatus comprising: a first image input unit constructed to input a first image; a partial region extraction unit constructed to divide the first image into a plurality of partial regions, and to set combined regions each of which includes two or more partial regions; a feature amount extraction unit constructed to extract feature amounts of the respective partial regions; a comparison unit constructed to compare the respective extracted feature amounts of partial regions of the first image and feature amounts of a second image which are registered in a database; and a determination unit constructed to determine a similarity between a feature amount of at least one combined region and a feature amount of a similar region of the second image, wherein the similar region is a circumscribing rectangle including two or more partial regions of the second image each having a feature amount similar to any one of the respective extracted feature amounts of partial regions of the first image.