1. A document group analyzing apparatus comprising: a processor comprising: a searching unit configured to locate a document specified by a searching condition in a document database that holds causally related documents; a related document group extracting unit configured to extract the document located by the searching unit, and all documents in the database that are casually related to the document located by the searching unit; a document group classifying unit configured to collect the documents extracted by the related document extracting unit, organize the collected documents into a group, and classify each document in the group by kind of document; a result outputting unit that provides a client with a result of the classification done by the document group classifying unit; and a state-transition graph generating unit configured to generate a state-transition graph, wherein nodes are connected in sequence by the causal relation, each node being associated with one of the kinds of the documents, and a state-transition key that goes from a certain node N 1 to another node N 2 is expressed by a document that belongs to the kind of the document associated with the node N 1 , a document group that belongs to each class generated by the document group classifying unit is made into an individual state-transition graph, and common portions in a transition pattern of individual state-transition graphs of the kinds of documents are unified.