PURPOSE: To enable to use terminal devices in a low speed in admixture with a transmission system with a high communication speed, by providing a simple conversion means to terminal devices of a slow communication speed.

CONSTITUTION: A bi-phase (BPS) signal from a data transmitter 17 is inputted to data demodulation sections 19, 22 at a data demodulation section in terminal devices 18, 21 for demodulation. The output in a data demodulation section 19 is inputted to a high speed processing section 20 with the same speed as the transmission speed of a transmitter 17, the output of the data demodulation section 22 is converted to a half speed through a conversion section 23 and inputted to a low speed processing section 24 having a half speed fo the transmission speed of a transmitter 17. Data processing is made with each speed at the high speed processing section 20 and low speed processing section 24.