A computer system with a monitor display, comprises a decoder unit configured to decode video data containing digital watermark information to derive decoded data, and configured to determine, based on information included in the video data, whether or not first certification for certifying that the video data is legitimately reproduced data succeeds, and a graphics controller which determines, using the digital watermark information, whether or not second certification for certifying that the video data is to be protected from being copied succeeds, the graphics controller converting the decoded data into a protected decoded data from being copied and outputting the protected decoded data to the monitor display if the first certification and the second certification succeed, and the graphics controller inhibiting the decoded data from being output to the monitor display if the first certification fails and the second certification succeeds.