1. A presentation system comprising: a screen device; a projecting device for projecting an image onto the screen device; an indication mark for setting two reference points, which is two orthogonal points on a rectangular virtual screen, near a presenter and for the presenter operating a point on the virtual screen; a photographing device for photographing the virtual screen and the indication mark; and an arithmetic operation device for generating coordinates of the virtual screen from the two reference points photographed by the photographing device, and for calculating coordinates of the indication mark on the generated virtual screen, wherein the projecting device projects a point image onto the coordinates of the indication mark on the image.