An optical crystal includes a first non-linear optical crystal that generates terahertz waves corresponding to a difference frequency component in incident light with two different wavelengths by a difference frequency generation, and a second non-linear optical crystal that generates terahertz waves corresponding to a difference frequency component in incident light with two different wavelengths by a difference frequency generation, the second non-linear optical crystal being different in material from the first non-linear optical crystal, and the first non-linear optical crystal and the second non-linear optical crystal being disposed in contact or close together.