In order to accomplish highly-efficient nonlinear optical wavelength conversion for terahertz waves by using a simple configuration, an embodiment of the present invention provides a terahertz wave generating device 100 provided with a pump light source 104 that generates pump light LP of a single wavelength and a nonlinear optical element (periodical polarization inversion element) 102. The nonlinear optical element has a periodic structure in which the polarization or the crystal orientation is periodically inverted with a certain inversion period Λ. When the pump light enters the nonlinear optical element, idler light LI and signal light LTHz are generated. The idler light and the signal light satisfy the law of conservation of energy for the pump light and a collinear phase matching condition for a virtual pump light wave vector k′p obtained by vector addition or subtraction of a grating vector kΛ which corresponds to the inversion period, to or from a pump light wave vector kp in the nonlinear optical element. The embodiment of the present invention also provides an optical parametric amplifier, a terahertz wave detector, and a nonlinear optical element.