To instantaneously detect frequency and phase in accurate and stable manner by excluding the effect of noises while adapting to fluctuating frequency as a frequency/phase detecting method for fundamental wave components of single-phase AC signal accompanied by frequency fluctuation.

In the fundamental wave component detecting method, the square phase signal having advancement or delay from a fundamental wave component of single-phase AC signal by /2(rad) phase is generated from the single-phase AC signal via a stable filter. Using at least a generated square phase signal, frequency and phase of the fundamental wave component or the frequency of fundamental wave component and phase cosine sine value are at least detected. The characteristics of a stable filter for generating square phase signals is allowed to be such characteristics as bandpass characteristics added with derivative characteristics or integral characteristics. Using the detected frequency, only the filter coefficient corresponding to the central frequency of the bandpass characteristics is adaptively adjusted.