PURPOSE:To reduce a fault caused by the thunder current immersed in a telephone circuit, by providing a relay contact through which the telephone circuit is connected to each key telephone during the telephone circuit is busy. CONSTITUTION:A telephone circuit is usually connected only to an incoming detecting circuit 3. A control circuit 8 receives the originating or answer signal given from a telephone for each telephone, and the telephone circuit is connected only when a relay 9 is recovered. In such way, each telephone is separated from the telephone circuit for 23 hours in a day since the telephone circuit is usually used for an hour per day. As a result, the telephone is protected from the thunder current by a contact 10 of the relay 9.