1. 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 retrieves 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 calculating a first number of all the associated documents, a second number of documents, within the associated documents, matching the retrieval expression, a third number of documents, within the associated documents, containing each of the objective keywords and matching the retrieval expression, and a fourth 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, the third number and the fourth 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, and determines a degree of association between the retrieval expression and each of the documents stored in the document information storing element based on the degree of similarity for each of the objective keywords obtained by the degree of similarity determining element and the associate document information.