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 the plurality of registration images stored in said storage portion, to detect an identical or similar image among the plurality of registration images, and to extract a common area including said image detected among the plurality of registration images; a search area determination portion to determine a non-common area among the plurality of registration images 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 being an image positioned in the location indicated by said search area information of a document image received from said image scanning apparatus with a reference image being an image positioned in said search area of each of the plurality of registration images stored in said storage portion; a transmission request portion to request transmission of the 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 the document image received from said image scanning apparatus as a registration image into said storage portion.