The present invention is adapted to make all the pixels of electrophoretic display 17 white (step S103) when light no longer incomes on the face of electrophoretic display 17 ("Yes" at step S102) . When someone places the face of electrophoretic display 17 on a contact glass of a copy machine to copy the contents on electrophoretic display 17 as they are, all the contents currently displayed on electrophoretic display 17 is deleted. As the result, the present invention can prevent the contents on electrophoretic display 17 from being easily copied by the copy machine as they are.