A terahertz wave generating apparatus includes an excitation light source for outputting an excitation light at a predetermined wavelength, an optical crystal being excited by an irradiation with the excitation light in order to generate a terahertz wave and terahertz wave amplifying means for repeatedly performing an optical parametric amplification for the terahertz wave by use of the excitation light, wherein the terahertz wave amplifying means includes an optical waveguide having the optical crystal serving as a core and a medium serving as a clad whose refractive index is smaller than a refractive index of the optical crystal, and the inputted excitation light is propagated within the optical waveguide with fulfilling a condition for a total reflection.