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°) was performed by supplying H atoms that were generated by the exposure of H2gas to a heated tungsten filament. After that, the surface was exposed to O2gas (1000 L) at room temperature. The magnitude change of two H‐adsorption components in the Si 2pspectra and their peak shifts show a two‐stage hydrogen adsorption process on the Si(100) surface. Oxidation of the monohydride phase (2×1‐H) is reduced to less than 1/3 of that of a clean Si(100) surface, while that of the dihydride phase (1×1‐2H) 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° was not more oxidized than the flat Si(100) surface. This suggests that atomic steps have little effect on oxidation.