The H-termination process on a Si(100) surface and the oxidation-inhibiting effects were investigated quantitatively by x-ray photoelectron spectroscopy using synchrotron radiation. The H-terminated Si(100) surface has two hydrogen-adsorption components (monohydride and dihydride). This article reports on the oxidation-inhibiting effects of each phase. The H termination of Si(100) surfaces (flat and tilted by 10-degrees) was performed by supplying H atoms that were generated by the exposure of H-2 gas to a heated tungsten filament. After that, the surface was exposed to 02 gas (1000 L) at room temperature. The magnitude change of two H-adsorption components in the Si 2p spectra and their peak shifts show a two-stage hydrogen adsorption process on the Si(100) surface. Oxidation of the monohydride phase (2 x 1-H) is reduced to less than 1/3 of that of a clean Si(100) surface, while that of the dihydride phase (1 X 1-2H) is reduced to below 1/9. Oxidation occurs more easily for the monohydride phase than for the dihydride phase. Further, the H-terminated Si(100) surface tilted by 10-degrees was not more oxidized than the flat Si(100) surface. This suggests that atomic steps have little effect on oxidation.