A thermal emission source that allows a wider range of material choices than those of conventional techniques, so that light having a desired peak wavelength can easily be obtained. A thermal emission source includes a thermo-optical converter composed of an optical structure in which a refractive index distribution is formed in a member made of an intrinsic semiconductor so as to resonate with light of a shorter wavelength than a wavelength corresponding to a bandgap of the intrinsic semiconductor. When heat is externally supplied to the thermo-optical converter, light having a spectrum in a band of shorter wavelengths than a cutoff wavelength is produced by interband absorption in the intrinsic semiconductor, and light of a resonant wavelength λr in the wavelength band, the light causing resonance in the optical structure, is selectively intensified and emitted as thermal emission.