The present invention is provided with a display (14) provided with a touch panel (16), so that when a menu key (22c), etc., is operated on while a lock state is set, a lock screen image is displayed on the display (14). A prescribed area (80a, 80b) is set on both sides of the lock screen image. When a touch operation is performed in the prescribed area, a temporary invalid area (IA) is set so as to include a touch area corresponding to the touch operation. Information indicating the temporary invalid area is stored as history data in a history table, and the extent of an invalid area (NA) is determined on the basis of a plurality of history data. Then, when the lock state is released, the invalid area thus determined is set.