When data is transmitted to a destination of data transmission, a multiplex communication system 1 as a source of data transmission receives, from the destination of data transmission through a data reception processing unit 15, the information about the amount of data which can be successively received, compares the amount of data which can be successively received with the amount of data which can be successively transmitted as preset, selects the smaller amount, creates a packet for successively transmitting data frames corresponding to the amount of data as selected, and transmit said packet through a data transmission processing unit 14.