TERMINAL ADAPTER

PROBLEM TO BE SOLVED: To provide the structure of TA capable of performing the switching operation of a line reliably by eliminating user's anxiety. SOLUTION: When a signal transmitted from a PB detection circuit 12 is the line switching signal of a specific number or mark fixed in advance, a hooking signal of a length required for switching speech among three tarties, e.g. is transmitted to an ISDN network 2. Further, a CPU 8 may detect the call-incoming signal of the speech among three parties, e.g. to transmit it only when a line switching signal is detected. A PB detection circuit 12 may compare the level difference between the level of a signal to the analog terminals 6 and 7 from an ISDN network 2 and the level of a signal to the network 2 from the terminals 6 and 7 with a signal level comparing part 12a and to make the CPU 8 transmit the line switching signals in the case of detecting the level difference of 8 dB fixed by the standard of ISDN. The circuit 12 is provided with a signal cut-off part 12b fir cutting off signals to the terminals 6 and 7 from the network 2 when a push button signal is detected.