A terahertz-wave element includes a waveguide (2, 4, 5) that includes an electro-optic crystal and allows light to propagate therethrough, and a coupling member (7) that causes a terahertz wave to enter the waveguide (2, 4, 5). The propagation state of the light propagating through the waveguide (2, 4, 5) changes as the terahertz wave enters the waveguide (2, 4, 5) via the coupling member (7).