The present invention provides a thermal emission source capable of switching the intensity of light at a high response speed similarly to a photoelectric conversion element. A thermal emission source 10 includes: a two-dimensional photonic crystal 12 including a slab 11 in which an n-layer 112 made of an n-type semiconductor, a quantum well structure layer 114 having a quantum well structure, and a p-layer 111 made of a p-type semiconductor are stacked in the mentioned order in the thickness direction, wherein modified refractive index areas (air holes 121) whose refractive index differs from the refractive indices of the n-layer 112, the p-layer 111 and the quantum well structure layer 114 are cyclically arranged in the slab 11 so as to resonate with a specific wavelength of light corresponding to a transition energy between the subbands in a quantum well in the quantum well structure layer 114; and a p-type electrode 131 and an n-type electrode 132 for applying, to the slab 11, a voltage which is negative on the side of the p-layer 111 and positive on the side of the n-layer 112.