The state of a label at an optional time point, such as a past time point, can be recognized, while the label can be regenerated even when the label has been deleted. When a label on which can be pasted an object, such as text, picture or sound, is acted upon (yes at step S141) the difference of the label information is computed (step S142) and the computed difference information is stored (steps S144, S145). The label state is then regenerated based on the time information stored in association with the label and the difference information. Also, time is displayed, plural time intervals are displayed (step S201), a desired one of the plural time intervals is selected (step S202) and time display is controlled responsive to the selected time intervals (steps S203 to S206), to facilitate retrieval of the information corresponding to the specified time.