PURPOSE: To attain the transmission of a digital signal having a fast transmission speed utilizing an analog transmission line having a considerably narrow frequency band by converting the digital signal from a digital transmission line to a false bipolar signal.

CONSTITUTION: In a building 31, plural terminals (a), (b) and (c) are connected with each other through line terminating sets G and H, and a repeater C, forming one network. Similarly, in an another building 32, plural terminals (d), (e) and (f) are connected with each other through a line terminating set I and a repeater D, forming another network. A transmission line 33 is the analog transmission line and converts a digital data to a false bipolar code, then transmits it. The digital data is converted to the false bipolar code by radio connection equipments A and B.