A waveform reconstruction device which reconstructs a time waveform of an input optical signal, the waveform reconstruction device comprising: an input-spectrum obtaining unit configured to obtain information indicating a power spectrum of the input optical signal; an output-spectrum obtaining unit configured to obtain, for each of intensities of the input optical signal, a measured power spectrum which is obtained by measuring an output optical signal that is the input optical signal output after having propagated through an optical transmission medium having a nonlinear optical effect; a phase-spectrum calculation unit configured to (i) simulate, for each of the intensities of the input optical signal where the input optical signal is assumed to have a given phase spectrum, propagation of the input optical signal through the optical transmission medium, to calculate a power spectrum of the output optical signal, and (ii) perform iterations of simulating the propagation to make a search for a phase spectrum of the input optical signal while changing the given phase spectrum to reduce differences between the calculated power spectra and the measured power spectra which are power spectra of output optical signals corresponding to the intensities of the input optical signal; and a waveform reconstruction unit configured to perform frequency-time transform on the phase spectrum found through the search by the phase-spectrum calculation unit and the power spectrum indicated in the information obtained by the input-spectrum obtaining unit, to reconstruct the time waveform of the input optical signal, wherein in the iterations of simulating the propagation through the optical transmission medium, the phase-spectrum calculation unit is configured to simulate the propagation based on, out of a nonlinear optical effect proportional to a nonlinear constant y of the optical transmission medium and a dispersion effect produced by high order dispersion that is at least second-order dispersion β 2 in the optical transmission medium, only the nonlinear optical effect, the propagation being simulated in disregard of the dispersion effect, characterised in that when making the search for the phase spectrum of the input optical signal, the phase-spectrum calculation unit is configured to change the given phase spectrum in accordance with a constraint on the phase spectrum that is to be satisfied when the input optical signal is assumed to be a resultant signal that has propagated through a medium having only the dispersion effect out of the nonlinear optical effect and the dispersion effect, and subsequently change the given phase spectrum independently of the constraint. an input-spectrum obtaining unit configured to obtain information indicating a power spectrum of the input optical signal; an output-spectrum obtaining unit configured to obtain, for each of intensities of the input optical signal, a measured power spectrum which is obtained by measuring an output optical signal that is the input optical signal output after having propagated through an optical transmission medium having a nonlinear optical effect; a phase-spectrum calculation unit configured to (i) simulate, for each of the intensities of the input optical signal where the input optical signal is assumed to have a given phase spectrum, propagation of the input optical signal through the optical transmission medium, to calculate a power spectrum of the output optical signal, and (ii) perform iterations of simulating the propagation to make a search for a phase spectrum of the input optical signal while changing the given phase spectrum to reduce differences between the calculated power spectra and the measured power spectra which are power spectra of output optical signals corresponding to the intensities of the input optical signal; and a waveform reconstruction unit configured to perform frequency-time transform on the phase spectrum found through the search by the phase-spectrum calculation unit and the power spectrum indicated in the information obtained by the input-spectrum obtaining unit, to reconstruct the time waveform of the input optical signal, wherein in the iterations of simulating the propagation through the optical transmission medium, the phase-spectrum calculation unit is configured to simulate the propagation based on, out of a nonlinear optical effect proportional to a nonlinear constant y of the optical transmission medium and a dispersion effect produced by high order dispersion that is at least second-order dispersion β 2 in the optical transmission medium, only the nonlinear optical effect, the propagation being simulated in disregard of the dispersion effect, characterised in that when making the search for the phase spectrum of the input optical signal, the phase-spectrum calculation unit is configured to change the given phase spectrum in accordance with a constraint on the phase spectrum that is to be satisfied when the input optical signal is assumed to be a resultant signal that has propagated through a medium having only the dispersion effect out of the nonlinear optical effect and the dispersion effect, and subsequently change the given phase spectrum independently of the constraint.