A method and apparatus for superimposing additional information onto image information in a manner such that the additional information is inconspicuous on the displayed image. The additional information is spread throughout the image signal via logical operation with a pseudo-random code sequence. A dithering process is performed on the spread additional information in which masking patterns are randomly applied to mask some pixels from superimposition, thereby reducing the effective level of the additional information signal. High frequency random noise is added to the dithered additional information to further reduce the perceptible effect of the additional information on the image. The dithered and noise-added additional information signal is superimposed with the image signal, and recorded on a storage medium.