A wavelength conversion element includes a crystal having a periodically poled structure in which polarization is inverted with an inversion period Λ along a z-axis which is an input axis of a light pulse. The wavelength conversion element is configured to generate an output the inversion period Λ(x) at each position x by change of the inversion period Λ according to the position x, and when a target frequency linearly changing with the position x is set to fT(x)=b+ax, a frequency width of the output frequency is set to δf(x), and the output frequency is set to f(x)=fT(x)+α(x), the output frequency is set to coincide with the target frequency within a range satisfying a condition |α(x)|≤δf(x).