1. A relevant document display apparatus for displaying a group of documents containing documents to be cross-referenced, comprising: a document group analysis device for classifying a plurality of documents forming a set of documents into one or more group of documents to be cross-referenced by analyzing a reference between the plurality of documents forming the set of documents, and for extracting information about classification as document group information; a document group keyword extraction device for extracting a keyword contained in a document forming part of each group of documents by referring to the document group information; and a document group keyword display device for displaying a title relevant to each group of documents and a corresponding keyword extracted from the group of documents by said document group keyword extraction device.