2. The document rating calculation system according to claim 1 , wherein for each item included in a document having a hierarchical structure of items, the item information database stores, as the mutual dependent relationship among the items, a subordinate item set which is a set of items included in a lower class in the hierarchical structure and including said each item as a top, and for each item fulfilling the condition item of the retrieval condition, the related item selection unit selects that item and the subordinate item set thereof as a set of related items.
3. The document rating calculation system according to claim 2 , wherein for each item included in a document having a hierarchical structure of items, the subordinate item set optionally including an arbitrary item besides a subordinate item included in a lower class in the hierarchical structure and including said each item as a top.