1. A document rating calculation system comprising: an item information database that stores a mutual relation among items into which a document is divided and a rating for each of the items which is calculated based on a predetermined criterion; document retrieval unit that retrieves a document fulfilling a given retrieval condition, and, for each condition item of the retrieval condition, specifies an item fulfilling the condition item in the retrieved document; related item selection unit that selects a set of related items for each item fulfilling the condition item of the retrieval condition and specified by the document retrieval unit in the document retrieved by the document retrieval unit, based based on the mutual relation among the items which is stored in the item information database; fulfilling-item set specifying unit that performs a logical operation of the retrieval condition between sets of related items selected by the related item selection unit to specify a set of items fulfilling the retrieval condition; and score calculation unit that calculates a document rating of the document fulfilling the retrieval condition based on the ratings of items stored in the item information database and included in the set of fulfilling items specified by the fulfilling-item set specifying unit.