Abstract: A method for flating the phase characteristic of a differential detection signal in one higher and lower regions thereof relative to its resonance frequency by controlling a differential characteristic. A searching, over a range, which is wider than the width of the flat region, is accomplished prior to phase characteristic being effected thereby discriminating a fundamental resonance frequency. After this discrimination of the fundamental resonance frequency, automatic tracking is effected under stable corrected phase characteristics for the fundamental resonant frequency whereby high electro-mechanical conversion efficiency can be maintained.