PROBLEM TO BE SOLVED: To detect the fundamental period of the harmonic wave-form with high accuracy by setting the fundamental period of the harmonic wave-form contained in the wave-form to aT when a period matching with a specific equation expressed by an optional period T and a specific integer (a) is detected among a series of periods. SOLUTION: The fundamental period of the harmonic wave-form contained in the wave-form is set to aT when at least two periods matching with aT/(a-b) or at/(a+c) are detected among a series of periods, where T is an optional period within a series of periods detected from the wave-form by the nonharmonic period detecting method capable of detecting the period of the sine wave generally in the nonharmonic relation forming the wave-form, (a) is an integer of 2-10, (b) and (c) are integers of 1-10, and ba. The period of the sine wave actually forming the wave-form or the sine wave approximate to it can be detected with high accuracy.
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