A method and apparatus for directly and indirectly preventing illegal copying of works such as cinema or video images displayed on a display apparatus such as a theater screen or display unit. In particular, the method and apparatus prevents illegal copying of a displayed work wherein one seeks to use an image pickup apparatus, such as a video camera, to make a copy of the displayed work. Prevention of illegal copying is achieved by implementing one or more changeover modes to changeover a display operation condition so that a stripe pattern or the like is produced on a redisplayed image to deteriorate the quality of the redisplayed image.