We propose a quantitative method of evaluating observers' perceptions, that is, "feeling of immersion," when a visual stimulus is displayed on a surround display. The proposed method uses the movement of an observer's center of gravity induced by a visual stimulus as an index. To analyze the quantitative relationship between body sway and visual stimuli, an ARX model was fitted to data obtained by measuring observers' body sway when a visual stimulus was displayed on surround displays. The results showed that adding horizontal screens increased the observers' body sway, whereas vertical screens did not when the visual stimulus moved in a rolling fashion. (C) 2003 Wiley Periodicals, Inc.