Excitation light of two wavelengths is incident to an optical crystal from a first face side, and a terahertz wave THzb is generated from a second face, and the excitation light that has passed through the optical crystal is reflected, made incident to the optical crystal from the second face side, and a terahertz wave THza is generated from the first face. Terahertz waves with similar characteristics to each other are thereby generated reliably in plural directions.