1. A document processing method of editing a document formed from a plurality of originals, wherein the document is stored in a tree structure of originals, said method comprising: a first display control step of controlling a process of displaying a graphic object representing a specific chapter in a source original such that the graphic object moves on a document in response to instructions to drag the graphic object, when moving or copying the specific chapter in the source original on a certain portion of the document; a recognition step of recognizing a boundary between chapters in a destination original in the document, near a position of a cursor for dragging the graphic object representing the specific chapter; and a second display control step, of controlling a process of displaying a first display screen, in which the tree structure is shown, and a second display screen, in which the tree structure is not shown, wherein said first display control step includes controlling a process of displaying a mark for indicating to which areas on both sides of the recognized boundary in the destination original the specific chapter is to belong, and said second display control step includes controlling a process of assigning a bar indicating a belonging node to a boundary at a higher level on the second display screen.