PURPOSE: To uniquely decide a target bearing for an output voltage by applying signals with a phase difference increased by a prescribed phase angle to a second phase difference detecting circuit or using the output thereof for an alarm utilizing the inversion of the second phase difference detecting circuit.

CONSTITUTION: Input signals are inputted to a phase difference detecting circuit 5 to be exchanged for an output voltage. In accordance with the output voltage, a sign discriminating circuit 8 produces a positive or negative output. A delay circuit 11 gives one of the input signals a time delay in accordance with a necessary phase difference. An input control circuit 6 changes over a changeover switch circuit 10 according to the output of the sign discriminating circuit 8. An output control circuit 7 turns on or off an output switch circuit 9 by the output of a sign discriminating circuit 8'. The output switch circuit 9 turns on or off the output of the phase difference detecting circuit 5 and the changeover switch the positively or negativity of the phase difference of the input signals. In the present embodiment, the quantity of phase delay is set to 80°.