An anti-aliasing filter is a low pass filter for preventing aliasing errors that may occur during sampling. Normally, the maximum frequency (fm) that is included in the input signal is unknown. Therefore, when frequencies are analyzed by the FFT analyzer, sampling frequency fs is determined, the power of frequency bands of 1/2 fs or more is sufficiently attenuated by the low pass filter that is indicated above, and then sampling is performed. The 1/2 fs frequency is referred to as a Nyquist frequency.