Analyzing a spectrum is a good way to estimate the frequency-components (input frequencies) of an input signal, because the spectrum is calculated by some functions that are maximum at input frequencies (A. Papoulis, Circuits and Systems: A Modern Approach, p.391, Holt, Rinehart and Winston, Inc., New York, 1980). The functions are called sampling functions or sinc functions: sin x/x. We describe an estimation method of closed frequencies by using the phase property of the functions that calculates a spectrum in this paper.