PURPOSE: To prevent a trunk being held in invalid by automatically restoring the trunk after the lapse of set time when a response signal is not sent from an opposite station after sending number information or when the subscriber of its own station is not restored after detecting a restoring signal from the opposite station.

CONSTITUTION: The trunk 2 makes a signal sending circuit 3 to be an ON-state so as to execute the operation of transmission and connection and sends the number information to the opposite station. After sending the number information, a time counting circuit 6 counts time until the response signal is detected. When a signal reception circuit 4 does not detect the response signal within the time set by a control circuit 7, the circuit 7 makes the circuit 3 to be an OFF-state and restores the trunk. Besides, when a call is finished and the circuit 4 receives the restoring signal from the opposite station, a time counting circuit 5 counts time until the subscriber of its own station is restored. When the subscriber of its own station is not restored after the lapse of the time set by the circuit 7, the circuit 7 makes the circuit 3 to be the OFF-state and restore the trunk 2.