An image-processing apparatus including computation means and rotation/parallel-shift addition means is provided. The computation means is configured to compute a parallel-shift quantity and a rotation angle of the observed screen. The rotation/parallel-shift addition means is configured to move the observed screen in a parallel shift according to the parallel-shift quantity computed by the computation means, rotate the observed screen by the rotation angle computed by the computation means, and superpose the shifted and rotated observed screen on the reference screen or a post-addition screen obtained as a result of superposing observed screens other than the observed screen on the reference screen in order to add the other observed screens to the reference screen.