1. An apparatus for expanding a character string, wherein the character string is entered to search image information of documents, the apparatus comprising: a character string dividing device to divide the entered character string into a plurality of partial character strings each having a plurality of characters; a referencing device to reference a similarity table, the similarity table previously storing groups of similar partial character strings, each of the groups of similar partial character strings being derived from each of the plurality of partial character strings obtained from the character string dividing device by changing at least one of the characters of each partial character string to a different character which is similar in shape; and an expansion device to combine the plurality of similar partial character strings given by the referencing device into expanded words and store them in an expanded word table.