A terahertz wave generator 1 includes a nonlinear crystal 4, which is capable of generating a terahertz wave TH, a pumping beam emitter 2, which emits a pumping beam L1, a seed beam emitter 3, which is disposed so as to be parallel to a nonlinear crystal 4 and emits a seed beam L2, a first total reflection mirror 17 and a second total reflection mirror 18, which successively reflect the seed beam L2 emitted from the seed beam emitter 3 to cause the seed beam L2 to be incident on the nonlinear crystal 4, and a convex lens 6, which collects the seed beam L2.