14. The document rating calculation system according to claim 13 , wherein the score calculation unit calculates the document rating based on a sum of ratings of items included in a set of related items fulfilling the retrieval condition and stored in the item information database, the reference score calculation unit calculates the reference rating based on a number of times the document has been referred, and the score integration unit calculates the integrated rating by multiplying values of the document rating and the reference rating that are respectively normalized with predetermined criteria by a weight, and adding the weighted values.
13. The document rating calculation system according to claim 1 , wherein the item information database stores a document reference history for each document, and the document rating calculation system comprises: reference score calculation unit that calculates a reference rating of a document based on the reference history of a document fulfilling the retrieval condition which is retrieved by the retrieval unit; and score integration unit that integrates the document rating calculated by the score calculation unit and the reference rating calculated by the reference score calculation unit to calculate an integrated rating of the document.