A terahertz wave generator comprising: a nonlinear crystal capable of generating terahertz wave based on a parametric effect, a pumping beam emitter that emits pumping beam, a seed beam emitter that emits seed beam, and a convex lens that is disposed on an optical path of the seed beam and collects the seed beam, the terahertz wave generator being configured so that the pumping beam and the seed beam are caused to be incident through an end surface of the nonlinear crystal to cause the nonlinear crystal to generate terahertz wave, the terahertz wave generator comprising: a first collimator configured to adjust the pumping beam emitted from the pumping beam emitter into a collimated pumping beam, a second collimator configured to adjust the seed beam emitted from the seed beam emitter into a collimated seed beam, the terahertz wave generator being characterized in that it comprises: a concave lens that is disposed between the seed beam emitter and the convex lens on the optical path of the seed beam, and light guiding means configured to direct the collimated seed beam through the concave lens to thereafter be incident on the convex lens in parallel with an optical axis of the convex lens, and wherein said light guiding means is configured to move the seed beam with the concave lens in a direction orthogonal to an optical axis of the convex lens; and wherein the collimated seed beam, a diameter of which is increased by the concave lens, is caused to be incident on the nonlinear crystal so as to be superimposed on the collimated pumping beam on the end surface of the nonlinear crystal. a first collimator configured to adjust the pumping beam emitted from the pumping beam emitter into a collimated pumping beam, a second collimator configured to adjust the seed beam emitted from the seed beam emitter into a collimated seed beam, the terahertz wave generator being characterized in that it comprises: a concave lens that is disposed between the seed beam emitter and the convex lens on the optical path of the seed beam, and light guiding means configured to direct the collimated seed beam through the concave lens to thereafter be incident on the convex lens in parallel with an optical axis of the convex lens, and wherein said light guiding means is configured to move the seed beam with the concave lens in a direction orthogonal to an optical axis of the convex lens; and wherein the collimated seed beam, a diameter of which is increased by the concave lens, is caused to be incident on the nonlinear crystal so as to be superimposed on the collimated pumping beam on the end surface of the nonlinear crystal. a concave lens that is disposed between the seed beam emitter and the convex lens on the optical path of the seed beam, and light guiding means configured to direct the collimated seed beam through the concave lens to thereafter be incident on the convex lens in parallel with an optical axis of the convex lens, and wherein said light guiding means is configured to move the seed beam with the concave lens in a direction orthogonal to an optical axis of the convex lens; and wherein the collimated seed beam, a diameter of which is increased by the concave lens, is caused to be incident on the nonlinear crystal so as to be superimposed on the collimated pumping beam on the end surface of the nonlinear crystal.