To extract a document relating to an operation-objective document to refer easily thereto, without carrying out prework such as classification/filing of the documents and correlation therebetween.

An operation history control part 114 generates history information of an document operation by a document operation device 130 to be stored in an operation history information storage part 115. A relating document processing part 116 collates the operation history information of the document operated by the document operation device 130 with operation history information of other document, as to one or more of items selected by a user, and extracts the document having the high possibility of correlation with the operation-objective document to transmit a list thereof to the document operation device 130. The user selects the document on the related-document list displayed on a display 132 to refer to a content thereof.