When light beams of two different wavelengths applied from an excitation light source (14) are made incident on a nonlinear optical crystal (12) having a unique nonlinear coefficient, the nonlinear optical crystal generates THz waves resulting from difference frequency generation according to the nonlinear coefficient that the crystal itself has and SHG waves in which the light beams of two different wavelengths have been wavelength converted in accordance with the nonlinear coefficient. The generated THz waves pass through or are reflected from a sample and are detected by a THz detector (20). The SHG waves are detected by a SHG detector (22). A control unit (24) acquires THz measurement values T from the THz detector, acquires SHG measurement values S from the SHG detector, and uses baseline THz measurement values TB and baseline SHG measurement values SB acquired without the sample to perform baseline correction using (T/S)/(TB/SB).
