In a digital data transmission system of the present invention, the total transmission rate which changes with the passage of time in a plurality of video/audio signals multiplexed with a plurality of digital data on a transmission path is monitored without allocation of individual transmission rates to the plurality of video/audio signals so that encoders are controlled only when the total transmission rate exceeds the maximum transmission rate. Further, the plurality of digital data are transmitted within a range of the residual transmission rate obtained by subtracting the total transmission rate of the plurality of video/audio signals from the maximum transmission rate in the transmission path, whereby real-time transmission of video/audio signals and transmission of digital data can be made consistent with each other on one transmission path without reduction in video/audio quality of the transmitted video/audio signals.