PURPOSE: To obtain a detecting circuit of simple construction, by a method wherein levels of outputs of two signals combined to be of a reverse phase and the same phase in a four-terminal hybrid are subjected to subtraction and amplification in a differential amplifier.

CONSTITUTION: Two different signals inputted to input terminals 1 and 2 are combined to be of a reverse phase and the same phase in a four-terminal hybrid 3 respectively, and a reverse-phase composite signal is outputted to a reverse- phase output end 3A, while the same-phase composite signal is outputted to the same-phase output end 3B. The signals are detected by level detectors 4 and 5 and outputted as DC voltages. These output voltages A and B have characteristics corresponding to a phase difference between the two input signals. Next, these voltages A and B are subjected to subtraction and amplification by a differential amplifier 6 and thereby an output voltage C is obtained. The output voltage C turns to be an output voltage having a prescribed relation to the two signals in accordance with the phase difference between them. The phase difference between the two signals can be detected therefrom, and a detection voltage is outputted from an output terminal 7.