A data processor includes: a signal input section to which video and audio signals are input; a compressing section for compressing/coding the video and audio signals to generate video and audio data; a stream assembling section, which divides the video and the audio data into a plurality of packets, and makes a plurality of data units, in each of which a video packet representing a fraction of the video data and an audio packet representing a fraction of the audio data are multiplexed together to generate a data stream composed of a plurality of data units; and a writing section for writing the data stream on a storage medium. The stream assembling section determines, at least by a video playback time, what video packets and audio packets are included in each data unit, and if a portion of audio data, which is associated with the video data stored in a predetermined data unit, is missing from the predetermined data unit, then copied data, obtained by copying partial audio data including at least that missing portion of the audio data, is put into the data stream.