The damage layer of a Si(100) surface induced by low-energy (300-600 eV) rare-gas (Ne, Ar, or Xe) ion bombardment was in situ analyzed by x-ray photoelectron spectroscopy (XPS). The XPS analysis showed that, irrespective of the ionic species, an amorphous Si (a-Si) layer with a thickness of 10-30 Angstrom was formed and saturated by the dose of 10(15) cm(-2). It also showed that the thickness increased linearly with the ion energy and Ne ions formed the thickest a-Si layer. The saturated thickness measured with the XPS analysis has been confirmed by cross-sectional transmission electron microscopy. Formation process of the a-Si layer under small dose of ions can be explained in terms of the Poisson process. As a result, the size of an a-Si region formed by single ion bombardment was estimated. The region, which is supposed to have a cylindrical shape, has a radius of 2-4 Angstrom and a height of 10-30 Angstrom for each ionic species and energy. In situ XPS analysis for the Si surface bombarded with ion beams revealed the growth process of the a-Si layer in addition to the dependence of the thickness on the ionic species, energy, and dose.