The current position of each node in a hierarchical structure is held, and the past position of each node in the hierarchical structure is held. The current position is displayed in correspondence with the past position in a first user interface. The past position is displayed in correspondence with the current position in a second user interface. When the past position displayed in the first user interface is designated, the second user interface displays the designated past position. When a current position displayed in the second user interface is designated, the first user interface displays the designated current position.