A data processor according to the present invention includes: a receiving section to receive a data stream including data of first primary video to be presented by switching pictures at a first vertical scanning frequency and data of first auxiliary video to be presented synchronously with the first primary video; and a converting section for converting the first primary video data and the first auxiliary video data into data of synthetic video to be presented by switching the pictures at a second vertical scanning frequency, which is different from the first vertical scanning frequency. The data stream includes timing information defining respective times to present the first primary video and the first auxiliary video. The converting section associates second auxiliary video, having the same contents as the first auxiliary video on a picture of the first primary video, with a picture of second primary video, having the same contents as the counterpart of the first primary video, thereby generating synthetic video composed of the second primary video and the second auxiliary video.