1. A method of measuring the temperature and a spectral factor of a specimen, comprising the steps of: spectrally separating a radiant flux from said specimen with respect to effective wavelengths for n (n.gtoreq.3) different channels; converting the intensities of the respective spectrally separated radiant fluxes into corresponding electric detection values; obtaining electric values for effective wavelengths for m (n.gtoreq.m.gtoreq.3) different channels selected from said electric values for effective wavelengths for n channels; approximating spectral factors with respect to said effective wavelengths for said m different channels with an (m-2)-th order polynomial representing the effective wavelengths, thereby obtaining the temperature of said specimen from said electric detection values for said effective wavelengths for said m different channels; and obtaining the spectral factor of said specimen from said obtained temperature and said electric detection values for said effective wavelengths for m channels.