1. An image search apparatus capable of communicating with an image scanning apparatus, said image search apparatus comprising: a storage portion capable of storing a plurality of registration images; a common area extraction portion to compare a plurality of registration images stored in said storage portion and to extract a common area including an identical or similar image among a plurality of registration images; a search area determination portion to determine a non-common area excluding said common area, as a search area; a search area information transmission portion to transmit search area information indicating a location of said search area in an image to said image scanning apparatus; a comparison portion to compare a partial image received from said image scanning apparatus with a reference image corresponding to said search area of each of a plurality of registration images stored in said storage portion; a transmission request portion to request transmission of a document image if, as a result of comparison by said comparison portion, a registration image in which said reference image is identical or similar to said partial image does not exist in said plurality of registration images; and a registration portion to, in response to a request by said transmission request portion, store a document image received from said image scanning apparatus as a registration image into said storage portion.