In flow injection analysis (FIA), the baseline noise of the detection signal is an important parameter that determines the detection sensitivity. In this study, we applied the Fourier analysis/inverse Fourier analysis of Microsoft Excel to the detection signal of FIA to decrease the noise signal. The Fourier analysis of the detection signal for a sample of 0.06 mg L⁻¹ NO₂⁻ showed many frequency components with amplitudes less than 0.04 mV. After removing these frequency components, inverse Fourier analysis was applied, and a smooth detection signal was obtained. Even with a sample of 2.50 mg L⁻¹ NO₂⁻, the noise signals were successfully removed using the Fourier/inverse Fourier analysis. The signal-to-noise ratio (S/N) of NO₂⁻ was improved by ca. 15 times. We concluded that removing noise signals by Fourier analysis effectively improves the S/N of FIA.