Abstract: The present invention separates radiation from an object by a polarization filter 3 into polarized light beams, causes one of the beams to enter a spectrum analyzer 7 through a first optical path, causes the other to enter the spectrum analyzer 7 through a second optical path, and measures the two-color ratio, while causes radiation of a blackbody 2 placed in a vacuum ultralow temperature thermostatic chamber 1 in a quasi-thermal equilibrium state at an ultralow temperature in vacuo to enter the polarization filter 3 through a third optical path, separates the radiation into polarized light beams, causes the beams to each enter the same optical paths as the respective optical paths for the radiation of the object, causes the beams to enter the spectrum analyzer 7, measures the two-color ratio, and accurately obtains the temperature of the object on the basis of these two two-color ratios.