A data processing device to create a header of a multiplexed stream including a joined video stream (Ms) which is generated by joining respective corresponding frames of a plurality of video streams, respective video streams having a picture signal encoded by a prescribed encoding method and to be individually displayed in divided regions of a display screen, the device comprising: a data size calculation unit (3, 4) configured to calculate data sizes of respective joined frames of the joined video stream (Ms), the respective joined frames to be generated when the respective corresponding frames of the plurality of video streams are joined to generate the joined video stream (Ms), based upon code amounts of encoded picture signals of the respective corresponding frames of the plurality of video streams, before the respective corresponding frames of the plurality of video streams are actually joined to generate the joined video stream (Ms); a header creation unit configured to create the header of the multiplexed stream including the joined video stream (Ms) generated by joining the respective corresponding frames of the plurality of video streams, before the joined video stream (Ms) is actually generated by joining the respective corresponding frames of the plurality of video streams, using information of the data sizes of the respective joined frames of the joined video stream (Ms), calculated by the data size calculation unit (3, 4); a joined video stream generation unit configured to generate the joined video stream (Ms) by actually joining the respective corresponding frames of the plurality of video streams, after generation of the header of the multiplexed stream by the header creation unit has been completed; and a multiplexed stream output unit configured to perform output processing of the header of the multiplexed stream and generation and output processing of the multiplexed stream including the joined video stream (Ms). a data size calculation unit (3, 4) configured to calculate data sizes of respective joined frames of the joined video stream (Ms), the respective joined frames to be generated when the respective corresponding frames of the plurality of video streams are joined to generate the joined video stream (Ms), based upon code amounts of encoded picture signals of the respective corresponding frames of the plurality of video streams, before the respective corresponding frames of the plurality of video streams are actually joined to generate the joined video stream (Ms); a header creation unit configured to create the header of the multiplexed stream including the joined video stream (Ms) generated by joining the respective corresponding frames of the plurality of video streams, before the joined video stream (Ms) is actually generated by joining the respective corresponding frames of the plurality of video streams, using information of the data sizes of the respective joined frames of the joined video stream (Ms), calculated by the data size calculation unit (3, 4); a joined video stream generation unit configured to generate the joined video stream (Ms) by actually joining the respective corresponding frames of the plurality of video streams, after generation of the header of the multiplexed stream by the header creation unit has been completed; and a multiplexed stream output unit configured to perform output processing of the header of the multiplexed stream and generation and output processing of the multiplexed stream including the joined video stream (Ms).