Annealing of a Czochralski Si(100) substrate in a gas flow in which H-2 is diluted to a concentration of 3% in He was investigated. The surfaces annealed at 900 degrees C or above showed well-developed terraces with mono-atomic steps that alternate between the straight A-step and the zigzag B-step configurations, whereas no morphological improvement occurred in the surfaces annealed at temperatures below 900 degrees C, as confirmed by atomic force microscopy. Reflection high-energy electron diffraction observations and Fourier-transform infrared attenuated total reflection spectra of the flat surfaces clarified that the surface Si atoms make dimers with their dangling bonds terminated by H atoms, i.e., the surface reconstruction is Si(100)2X1/1X2-H. We show that the depth of the etching was not sufficient to account for the observed smoothening of the surface during the annealing at high temperatures, and conclude that the main mechanism of surface flattening is surface migration of the Si atoms.