a display device which has a zooming function and is required to display a logical screen larger in size than a physical screen , is operated in the following manner . when an action for indicating a zooming command is taken , a first rectangular frame similar in shape to the logical screen , a second rectangular frame similar in shape to a logical area which is now displayed on the physical screen , and a third rectangular frame similar in shape to a logical area which is to be subjected to a zooming operation , are all displayed on the physical screen , and moreover the third rectangular frame can be moved to a desired position by means of a coordinate input device such as a mouse . hence , the zooming operation and a desired logical area corresponding to the third rectangular frame placed at the desired position can be specified by a one - touch operation . thus , the operation procedure becomes simple , and the operability of the display device is improved .