PURPOSE: To interrupt a telephone set in a talking state automatically without malfunction in response to the case when a telephone set of an opposite party is hooked on.

CONSTITUTION: A hybrid circuit 8 receives a busy tone signal comprising interruption signal interrupted for plural number of times via a telephone line 10 and outputs the busy tone signal to a speaker 2, outputs a signal from a microphone 4 to the telephone line 10 and outputs a part to the speaker 4. A normally closed switch 20 is provided between the hybrid circuit 8 and the microphone 2. A busy tone frequency detection circuit 14 detects a signal with the same frequency as that f the busy tone and when it is detected twice consecutively, a time extract circuit 16a opens the normally closed switch 20. The busy tone frequency detection circuit detects the signal with the same frequency as that of the busy tone signal even after the switch is open and a control circuit 18 generates a call termination signal when it is detected thrice in total before the opening of the switch 20.