To eliminate/moderate a bottle neck of a communication I/F part on the premise that a data terminal side is configured as it is as much as possible.

In the case of sending data from a data terminal 1, a data companding section 11 of the data terminal 1 compresses transmission data and outputs the compressed data through a communication I/F 3 such as an RS 232C by way of a communication circuit section 12. A line connector 2 side uses a data companding section 21 to expand the compressed data received from the communication I/F3 through a communication circuit section 22 and outputs the data to a communication line 4 such as a public line or an ISDN through a line connection section 23 as usual transmission data. Conversely in the case that the data terminal 1 receives data, the line connection section 23 of the line connector 2 uses the data companding section 21 to compress the data received from the communication line and outputs the compressed data through the communication I/F 3 by way of the communication circuit section 22. The data companding section 11 of the data terminal 1 side expands the compressed data received from the communication I/F 3 through the communication circuit section 12 and processes the data as normal reception data.