A data transmission method for transmitting compressed data is disclosed. The method includes: classifying transmission target files into transmission groups; calculating, for each of the transmission files, a first compression time taken to compress the file by a first compression system and a first transmission time taken to transmit the file after being compressed by the first compression system; and determining, for each of the transmission groups, transmission order of files belonging to the transmission group based on the first compression time and the first transmission time.