When a main category change command is received, a main category change section refers to a category information storage section to change setting from a main category under current setting to another main category according to a predetermined order. After completing the main category change processing, a sub category change section refers to the category information storage section to select one sub category from among sub categories belonging to the newly set main category by use of random numbers. An image data generation section reads displayed information corresponding to the set main category and corresponding to the set sub category from a main-category expression-information storage section and a sub-category expression-information storage section, and generates image data according to the read displayed information.