1. A document retrieval system for retrieving documents from documents registered in a document database responsive to a retrieval condition designated by a user, said document retrieval system comprising: a) query converting means for converting said retrieval condition designated by the user into a query having a predetermined normal form in which keywords and at least one type of logical operation out of logical operations AND, OR and NOT are connected; b) bibliographical information indicating means for indicating at least information concerning which keywords correspond to each document of the documents registered in said document database; c) a keyword connection table having relationship values, each of the relationship values representing a degree of relationship between keywords; d) ranking means for ranking documents in accordance with relevance values, each relevance value indicating a degree to which a document fulfills the retrieval condition corresponding to the query, each of said relevance values being calculated for the document using the relationship values provided in said keyword connection table, wherein connected keywords are obtained with reference to the information indicated by said bibliographical indicating means as (1) keywords corresponding to the document and (2) keywords included in the query obtained by said query converting means; e) outputting means for outputting, as a retrieval result, the documents ranked by said ranking means; f) inputting means for inputting evaluation information indicating a degree to which each of the documents output by said outputting means is relevant to a document required by the user; and g) learning means for modifying one or a plurality of said relationship values in said keyword connection table based on the evaluation information input by said inputting means; wherein said query converting means includes first means for generating a first query in a conjunctive normal form, said first query in the conjunctive normal form having subqueries which are connected with each other only by logical AND operations, each of said subqueries being expressed by at least one keyword connected by at least one of logical OR and NOT operations; and wherein said ranking means includes: 1) first calculation means for calculating a sub-relevance value for each of said subqueries, said sub-relevance value indicating a degree to which each document fulfills each of said subqueries; and 2) second calculation means for calculating a relevance value for each of the documents using said sub-relevance value calculated for each of said subqueries by said first calculation means.