1. A method for identifying at least one unique character string in an input document which is input into a computer system, said computer system being operable to search one or more documents which are searchably stored in a storage medium, and said unique character string is used as a search string, the method comprising: associating and managing position information for a position in said searchably stored documents where one or more partial comparison document character strings are extracted from said searchably stored documents; extracting a partial input character string from said input document, and determining whether said partial input character string is a candidate character string; identifying a partial comparison document character string which matches at least a part of said candidate character string with a predetermined similarity factor or higher; identifying position data associated with said partial comparison document character string which matches with said predetermined similarity factor or higher; and recognizing said candidate character string as the unique character string by comparing appearance frequency information of at least a part of said candidate character string appearing in said input document with the position data and evaluating an amount of feature of said candidate character string.