An analog-to-digital converting method for converting an analog signal into a digital signal is provided, wherein the analog-to-digital converting method includes a measuring step of measuring a frequency characteristic of each of analog-to-digital converters in advance, a sampling step of sampling the analog signal, a Fourier transformation step of performing Fourier transformation on the data sampled by the plurality of analog-to-digital converters and generating a plurality of frequency domain signals, a correcting step of converting each of the frequency domain signals into a signal to be acquired in case that the frequency characteristic of corresponding analog-to-digital converter is ideal, by multiplying each of the frequency domain signals by a correction coefficient determined on the basis of the frequency characteristics of all the analog-to-digital converters, and a synthesizing step of synthesizing each of the frequency domain signals acquired at the correcting step and generating a frequency spectrum of the digital signal.