We present a method of the polarization degree analysis of exoplanets' objective-prism spectra. The polarization
analysis of the objective spectra can be used for discerning planet signal from noisy stellar light. The light reflected from
the planet is expected to be partially polarized, while the direct stellar light can be considered to be unpolarized. For
measuring objective spectra we use a four-quadrant polarization mask (FQPoM) coronagraph and a prism. The primary
suppression of starlight is achieved by destructive interference of the light passing through the central region of FQPoM.
For further suppression of starlight we use a polarization differential technique. By taking the difference between two
orthogonally polarized components of incoming light we can further suppress unpolarized starlight and reveal the
spectrum of the exoplanet. However, when the intensity contrast between the star and its planet is high, the starlight
noise impedes detection of the planetary spectrum. The analysis of the degree of polarization relieves the separation of
the planetary spectrum from the stellar noise. Moreover, any peculiar features in the objective spectra would be useful to
find out the location of the exoplanet. We obtained the experimental results under an intensity contrast of 3.5×10<sup>-5</sup> and an
angular separation of 4.9 λ/D.