The invention provides a cancel/undo method and system wherein the undo data amount upon changing of a tree structure is compressed and a cancel operation and an undo operation are united to significantly reduce the memory consumption and the processing time. A cancel data list and a undo data list are prepared, and undo data are classified into cancel type undo data and uncancel type undo data. Changed content information of tree structure data includes differential information of a changing object node position and a lower hierarchy side tree structure of the changing object node. Upon do operation, contents of the change are added as cancel data to the cancel data list while a cancel type is added to the undo data list. When cancel type data is object data of a cancel operation or an undo operation, a tree structure is restored with the last data of the cancel data list, and the data and contents of the change by the operation are added to the undo data list. When data of the uncancel data is object data of an undo operation, the tree structure and the cancel data are restored with the data of the uncancel type.