Our research group has developed a terahertz light source with high luminance and broadband, and is developing research on non-destructive inspection technology. As a nondestructive inspection technology for hot-dip galvanized steel sheets, we have built a terahertz spectrum database for corrosion products that may exist there. In this research, we aim to newly obtain spectral data of β-FeOOH. Absorption Spectrum is shown in Figure.1. Optical system is shown in Figure.2. The terahertz wave was generated by difference frequency mixing using a GaP crystal which is a nonlinear optical crystal. The standard powder sample was diluted with polyethylene powder and pelletized, and it was measured at room temperature by a permeation method. As a result of spectroscopic measurement, an absorption peak was obtained around 3.06 THz. On the day of the presentation, in addition to discussions on attribution of peaks, report together with ATR-FTIR results.