A crosstalk estimation system includes: a light source unit that generates a polarization-multiplexed light, which is a polarized light having multiplexed polarizations, of each wavelength in a sideband of a modulated signal and emits the polarization-multiplexed light of each wavelength; a multiplexer that multiplexes the modulated signal with the polarization-multiplexed light for each core, which is associated with one of the wavelengths; a transmission line that transmits the modulated signal multiplexed with the polarization-multiplexed light of each wavelength through a different core; a separation unit that separates the polarization-multiplexed light from the modulated signal multiplexed with the polarization-multiplexed light for each core; a measurement unit that generates light intensity data on the polarization-multiplexed light of each wavelength; and an estimation unit that estimates a crosstalk between the cores based on a difference in light intensity between the polarization-multiplexed light of the wavelengths.