PURPOSE: To perform data communication via a network controller, even with the connection to either subscriber telephone lines or exclusive lines, by providing a means which switchingly connects the subscriber telephone lines and the exclusive lines, for a DC circuit of the line side of a network controller.

CONSTITUTION: In a line switching device added to a subscriber telephone line data terminal device having a network controller NCU, e.g., an FAX 20, and connected to subscriber telephone lines LN and exclusive lines PL, a DC circuit of a line side of the device NCU is provided with a circuit 30 switchingly connecting the lines LN and PL. Data communication can be done via the device NCU even with the connection to a subscriber telephone line telephone set 13 and a telephone set 14 for exclusive lines. Thus, in switchingly connecting the subscbiber telephone lines or the exclusive lines, the NCU is used in common for the connection of the exclusive lines. Further, at the transmission the line switching is performed manually and at the reception, automatically, allowing to reduce the cost and to simplify the operation without remolding the FAX.