A terminal control system is provided that has a main control device, a plurality of terminal devices controlled by the main control device, and communication lines for connecting the main control device and each of the terminal devices so as to allow bidirectional communication, wherein the configuration is simple, communication time is short, and highly reliable bidirectional communication is possible. The terminal control system comprises a main control device 1, a plurality of terminal devices 2 controlled by the main control device 1, and communication lines 3 for connecting the main control device and each of the terminal devices so as to allow bidirectional communication, wherein control data is simultaneously and collectively sent from the main control device 1 to a plurality of the terminal devices 2, and terminal data for each of the terminal devices is independently sent from each of the terminal devices 2 to the main control device 1.