The marker has thereon one three-dimensional indicator 3. Based on the location and height of the three-dimensional indicator 3 relative to the plane indicator 2 as well as the estimated position of the camera relative to the marker, the center positions of the upper face and the lower face of the three-dimensional indicator 3 on the image of the camera are estimated. The estimated center position of the upper face of the three-dimensional indicator 3 is compared with that detected from the image. When the error between them has a value equal to or higher than a predetermined value, based on the posture estimated, a rotational transformation is carried out.