To provide a phase difference measuring circuit, applicable to low frequency, having high-accuracy over 360 degrees, the identical phase timing can be obtained if a received signal intensity varies, reducing the higher harmonics noise influence by generating a phase timing pulse at the timing of a polarity conversion, aiming the phase difference measurement between oscillated sine wave as periodic alternating current waves and received sine waves, having different amplitude.

Sine wave received is converted into a sinusoidal half wave by each half period. When (the timing which changes the polarity of the received sine wave) = (vertex of the sine half wave), VBE voltage of the connected transistor goes into an equilibrium state to a full-wave rectifying diode connected to a transformer, then the base current is generated by a loading coil only at the timing moment. The base current pulse is extracted as a voltage at a collector. This phase timing pulse rises in front of a zero-cross point, then falls after the zero crossing. The position of the wave front becomes the conversion point, where (zero cross point of the received wave) = (polarity conversion point of the sinusoidal half wave).