1. An intensity-interferometric spectral-domain tomographic imaging method for generating a tomographic image of a measured target based on information about interference light between measurement light and reference light, comprising: providing a light source, a reference mirror, a demultiplexer/multiplexer, a spectroscope element, a detector, a computer (including a display device), and a means for generating a phase difference π between the measurement light and the reference light; forming a spectrometer of the spectroscope element and the detector; separating light emitted from the light source using the demultiplexer/multiplexer; acquiring, using the spectrometer, information about spectral interference fringe intensity of interference light obtained by multiplexing measurement light obtained by one separated light being incident on and reflected from the measured target and reference light obtained by the other separated light being reflected by the reference mirror again using the demultiplexer/multiplexer to interfere; subsequently, acquiring, using the spectrometer, information about spectral interference fringe intensity of light obtained by multiplexing the measurement light and the reference light which is shifted by a phase difference π on the light travel path on the basis of the means again using the demultiplexer/multiplexer to interfere; and performing a Fourier transform calculation based on information about the two spectral interference fringe intensities acquired in a time-series manner and generating the tomographic image of the measured target from a position of a real part of a complex signal obtained as a result of the Fourier transform calculation which appears at a twice distance of a position of an imaginary part of the complex signal.