1. A document retrieving method in a document management system which manages documents, using a computer, comprising the steps of: (a) managing classifications of documents in accordance with a plurality of node trees having nodes linked in a hierarchical structure; (b) obtaining target nodes inputted by an operator; (c) when a particular one of the plurality of node trees is designated, OR-merging documents related to all the nodes ranging from documents linked to the target node to documents linked to lower nodes in the node tree which are linked to the target node directly or indirectly to create a collection of documents; (d) when a new different node tree is designated, OR-merging documents related to all the nodes ranging from documents linked to the target node to documents linked to lower nodes in the new node tree which are linked to the target node directly and indirectly to create another collection of documents; and (e) repeating the steps (b) and (d) and performing an AND-merge operation on the created collections to create and output a final collection of documents.