PURPOSE: To improve the degree of freedom of communication by switching automatically line switching/packet switching in data communication using an ISDN network.

CONSTITUTION: In a terminal adapter 10 constituted of a speed matching part 1 to be used in the line switching connection of the ISDN, a PAD (packet assembling/disassembling function part) 2 and a D channel protocol part 3 to be used in packet switching connection, command processing parts 4, 5, DTE interface parts 6, 7 connected to local terminals 11, 12, a DTE switching part 9 for switching connection between the speed matching part 1 or the PAD 2 and the DTE interface parts 6, 7, and an S/T bus interface part 8 connected to the ISDN network, the DTE switching part 9 is made to switch the connection to the speed matching part 1 or the PAD 2 by consistency information sent through the local terminals 11, 12 or the ISDN network.