PURPOSE: To obtain innumerable kinds of patterns by overlaying a parallel line film on another parallel line film with the angles changed.

CONSTITUTION: The parallel lines 2 composed of a large number of thin full lines are depicted on a transparent film to form a parallel line screen 1, and this screen 1 is used for replacing dots, and a number of the lines 2 is, for example, 150 or 175 per inch, or the like, depicted so densely that the presence of the lines is hardly recognizable, and confusion of vision is caused. Each of the lines 2 has various width, like 5, 10, 15%, and so on in terms of the areal ratio of the line to the white ground. When the screen 1 is overlaid on another screen 1, interference patterns, generally called 'moire', appear. These patterns vary in accordance with deviations of both directions of the liens from a slight angle to about 15° to give innumerable kinds of patterns.