1. A method of retrieving a document from a database, comprising the steps of: extracting a plurality of words contained in a query document received; collecting a plurality of words contained in a plurality of documents registered previously in the database for thereby creating retrieving indexes on the basis of numbers of times said plural words as collected occur in said previously registered documents, respectively, said retrieving indexes being held in a memory; calculating weights of said plural words, respectively, acquired in said extracting step through comparison with the words included in said retrieving indexes; selecting a plurality of words on the basis of weight values of said plural words as the condition for selection; and calculating degrees of similarity of said plural documents registered previously to said query document on the basis of said plurality of selected words.