Abstract: Automatic signal transfer apparatus for mobile phone is composed of an interface circuit, a transmitter/receiver signal transfer circuit, a DC power supply circuit, a line state detection circuit, a change over control circuit, a change over device, and a ringing circuit. With this structure the apparatus is able to automatically identify whether the input signal comes from the commercial telecom line, or from the mobile phone, and automatically transfers to the indoor telephone. By so, in order to receive both calling coming from the telecom line and calling to the mobile phone, and transfer the callings to the indoor telephone set without going through a tedious presetting procedure, it is only required to connect the mobile phone to an interface circuit. With such scheme, the apparatus of the present invention will never affect function and operation of the indoor telephone.