We used a multiwall carbon nanotube (MWNT) as a minute heater and studied structural changes in amorphous silicon (a-Si) supported on the surface of the MWNT heater during its Joule heating by in situ transmission electron microscopy combined with an optical spectroscopy system. During Joule heating of the MWNT at a temperature higher than 1690 K, thermal radiation spectra which follow Planck's blackbody radiation law started to be measured, and it was demonstrated that the MWNT heater could reach a maximum temperature of approximately 2135 K. The increase in temperature caused the crystallization of a-Si and subsequent reaction of crystallized Si with carbon in the outer layers of the MWNT to form Si carbide nanoparticles. At elevated temperatures between 2040 and 2135 K, the formed Si carbide nanoparticles thermally decomposed to generate carbon nanocapsules consisting of multilayered graphene shells.