A waveform reconstruction device (140) includes: a phase-spectrum calculation unit (143) which (i) simulates, for each intensity of an input optical signal assumed to have a given phase spectrum, propagation of the input optical signal through an optical transmission medium, to calculate a power spectrum of an output optical signal, and (ii) performs iterations of simulating the propagation while changing the given phase spectrum to reduce differences between calculated power spectra and measured power spectra of the input optical signal having the intensities, to search for a phase spectrum of the input optical signal; and a waveform reconstruction unit (144) which reconstructs a time waveform of the input optical signal using the phase spectrum found through the search, wherein the phase-spectrum calculation unit (143) changes the given phase spectrum or simulates the propagation, based on a nonlinear optical effect or a dispersion effect.