PURPOSE: To enable detection with high accuracy by converting the phase of one exciting wave, out of two-phase exciting waves, to the same phase as the phase of the other exciting wave, detecting the different point of a waveform, smoothing the detected waveform and converting it to a voltage in proportion to amplitude difference between the two phases.

CONSTITUTION: A phase conversion part 31 converts A and B waves, which are sent from a reference exciting wave generation part 11, to an A' wave of the same phase and transmits it to a waveform comparator part 32. The comparator part 32 compares the A' wave sent from the conversion part 31 and the B wave sent from a phase conversion part 13, detects the difference between the respective waveforms and sends a C wave to a detection part 33. The conversion part 1 sends out the B wave to a timing generation part 34 as well and the generation part 34 sends out a timing signal (t) based on the transmitted B wave to the detection part 33. According to the signal (t) from the generation part 34, the detection part 33 repeats detection, converts the C wave, which is sent from the comparator part 32, to a D wave and sends it out to a conversion part 35. The conversion part 35 smooths the D wave, which is sent from the detection part 33, converts the wave to a voltage V in proportion to the amplitude difference, and sends it out to an amplitude control part 12 and the amplitude is controlled.