1. A document rating calculation system comprising: a first information processing apparatus including, an item information database that stores a mutual dependent relationship based on topics represented by each item, among the items into which a document is divided and a rating for each of the items which is calculated based on a predetermined criterion; a document retrieval unit that electronically 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; a related item selection unit that, for each condition item of the retrieval condition, i) specifies an item related to the item fulfilling the condition item for each item fulfilling the condition item and specified by the document retrieval unit in the document retrieved by the document retrieval unit, based on the mutual dependent relationship among the items which is stored in the item information database, and ii) selects a set of related items including the item fulfilling the condition item and the item specified and related to the item fulfilling the condition item; a 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 a 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, wherein the item information database stores a value calculated based on a predetermined degree of account for a number of elements included in an item of the document and a type of the elements, as the rating of the item, wherein the types of elements included in the items of the document include any combination of a sentence, a figure, a table, an equation, an emphasis expression, a citation and a key word, and the item information database stores a value calculated based on a sum of product of a number of elements for each of the types of the elements included in the items of the document, a predetermined index for each of the types of the elements, and a predetermined weight set for the index, as a rating for each of the items.