Since most biomolecules have a specific molecular vibration mode in the mid-infrared (MIR) wavelength range, they absorb the MIR light energy of a specific wavelength. An MIR laser can be used to analyze several optical characteristics of biomolecules. The free electron laser (FEL) at Osaka University can be continuously varied in a wavelength range of 5.0-20.0μm. The pulse width is-5ps, and its peak power with 5 MW is higher than common laser systems. In this study, we measured the reflectance spectra of dentin and gelatin using FEL to estimate applications as a spectroscopic light source. Reflectance spectra resembled the transmission spectra by the Fourier Transform Infrared Spectrometer (FT-IR) method.