In a data transmission apparatus for transmitting digital video and audio data, a block forming circuit forms a plurality of data blocks of transmission data composed of at least one of digital video data, digital audio data, and digital auxiliary data related to the digital video data and the digital audio data. Thereafter, a grouping circuit groups a plurality of data blocks outputted from the block forming circuit so as to form transmission group data of one transmission unit block having a time length of approximately one three-hundredth second, and then transmits the transmission group data. Further, a classifying circuit preferably classifies the transmission group data into a plurality of sections, by adding section identification data to each transmission group data.