A video data transfer control system allowing secure protection of video data from unauthorized copying is disclosed. A scrambling key used to scramble video data is stored in a key register and a new scrambling key is stored in a register. At the timing of a vertical sync signal of the video data, the scrambling key is changed to the new one. As a result, the scrambling key can be frequently changed while transferring the video data, achieving enhanced protection of the video data from unauthorized copying.