At the time of copy & paste, an item, a size and a paste content are recorded, and the importance of the paste is determined based on the number of pasted items, and a pasted data size relative to the overall file. Also, an item name and a change content are recorded, and when the manager inputs an important keyword (or unimportant keyword) in visualizing a file history, the importance is increased (or decreased) if the keyword is included in the item name or the change content. A function of displaying the file history in order of importance if the file history is displayed in table format in visualizing the file history, and a function of displaying, or highlighting, only the source file in which the importance is greater than or equal to a threshold set by the manager if the file history is displayed graphically are provided.