“Moving pictures on sliding doors” date back to the Edo Era in Japan. These pictures were called moving pictures because they seem to move when they are viewed while moving. The aim of this study was to determine the mechanism underlying this illusionary phenomenon. For that purpose, photographs of a “moving picture” on sliding doors were taken using a digital camera, and the characteristic points on each image were digitized and stored in a computer. Then changes in perception of the picture with movement of the viewpoint of the observer were computed by using a coordination transformation technique. The results of calculation revealed that the picture had been painted from a bird's-eye view and that the illusionary motion is seen when the observer views the picture from an oblique angle while moving along the length of the sliding doors. When viewing the picture from an oblique angle while moving, distances from points on the picture to the observer's viewpoint change, and these changes in distances give rise to the illusionary motion of the picture. Thus, the mechanism by which the picture is perceived as moving is motion parallax.