7. An associate document retrieving apparatus that retrieves a document related to an inputted retrieval condition, comprising: a document information storing element that associates each of a plurality of documents with a keyword extracted from the document and stores the associated documents; a retrieval expression obtaining element that receives a retrieval expression; a number of documents calculating element that specifies a plurality of objective keywords from within the extracted keywords stored in the document information storing element, and calculates a first number of documents, within the associated documents, matching the retrieval expression, a second number of documents, within the associated documents, containing each of the objective keywords and matching the retrieval expression, and a third number of documents, within the associated documents, containing each of the objective keywords; a degree of similarity determining element that determines a degree of similarity between the retrieval expression and each of the objective keywords based on a relationship between the first number, the second number and the third number for each of the objective keywords; and a degree of association determining element that obtains associate document information of a document containing any of the objective keywords as the extracted keyword from the document information storing element and determines a degree of association between the retrieval expression and each of the associated documents based on the degree of similarity for each of the objective keywords and the associate document information.
7. An associate document retrieving apparatus that retrieves a document related to an inputted retrieval condition, comprising: a document information storing element that associates each of a plurality of documents with a keyword extracted from the document and stores the associated documents; a retrieval expression obtaining element that receives a retrieval expression; a number of documents calculating element that specifies a plurality of objective keywords from within the extracted keywords stored in the document information storing element, and calculates a first number of documents, within the associated documents, matching the retrieval expression, a second number of documents, within the associated documents, containing each of the objective keywords and matching the retrieval expression, and a third number of documents, within the associated documents, containing each of the objective keywords; a degree of similarity determining element that determines a degree of similarity between the retrieval expression and each of the objective keywords based on a relationship between the first number, the second number and the third number for each of the objective keywords; and a degree of association determining element that obtains associate document information of a document containing any of the objective keywords as the extracted keyword from the document information storing element and determines a degree of association between the retrieval expression and each of the associated documents based on the degree of similarity for each of the objective keywords and the associate document information. 11. The associate document retrieving apparatus as set forth in claim 7, further comprising: a retrieval expression transforming element that transforms the retrieval expression received by the retrieval expression obtaining element into a retrieval expression of a conjunctive normal form of a plurality of disjunctive clauses, wherein the number of documents calculating element calculates the first number, the second number and the third number for each of the disjunctive clauses included in the retrieval expression, the degree of similarity determining element determines the degree of similarity between each of the disjunctive clauses and each of the objective keywords, and the degree of association determining element determines the degree of association between the retrieval expression and each of the associated documents based on the degree of similarity between each of the disjunctive clauses and each of the objective keywords, and the associate document information.