PROBLEM TO BE SOLVED: To provide a terahertz-wave generating element including an electro-optic crystal, capable of modulating a generated terahertz wave at a relatively high speed.SOLUTION: The terahertz-wave generating element: includes a waveguide including an electro-optic crystal 4, that propagates light; an optical coupling member 7 which extracts a terahertz wave is generated from light propagating through the waveguide, to a space; and at least two electrodes 6a and 6b. The electrodes 6a and 6b has a function of changing a propagation state of the light propagating through the waveguide, by a first-order electro-optic effect in the electro-optic crystal 4 by applying an electric field to the waveguide. A crystal axis of the electro-optic crystal 4 of the waveguide is set such that the terahertz wave generated by a second-order nonlinear optical process and the light propagating through the waveguide are phase-matched.