1. A document information search method for retrieving a document containing a specific character string on the basis of a plurality of search requests issued from a plurality of search request sources, each of the plurality of search request sources including a search object and issuing a search condition, the method comprising the steps of: storing a plurality of search requests received from said plurality of search request sources; and, executing a search processing for said stored plurality of search requests by: logical-OR-combining each search object of each of said plurality of search request sources to generate a first combined search object; searching said first combined search object to locate a plurality of documents satisfying each search condition of each of said plurality of search request sources; dividing the plurality of documents into a plurality of sets of documents each of said plurality of sets corresponding to a one of said plurality of search request sources; and, logical-AND-combining each of said plurality of sets of documents with a corresponding one of said search objects to generate a search result for each of said plurality of search requests.