Initial adsorption dynamics of O₂ on Si(111)-7×7surface at room temperature was investigated by a supersonic molecular beam technique in conjunction with real-time X-ray photoelectron spectroscopy using synchrotron radiation. The initial sticking probability and the saturation coverage as a function of the translational kinetic energy of impinging O₂ were measured in the wide range from 0.03 eV, namely thermal gas generated by back-filling condition, to 2.3eV. We proposed a trapping-mediated process as a dominant adsorption mechanism at low kinetic energy region, while at high kinetic energies approximately over 0.06eV the direct adsorption overlapping the trapping-mediated one whose contribution became small with increasing incident energies were expected to be a dominant adsorption mechanism. A gradual increase of the saturation coverage with increasing kinetic energy was clearly observed in the incident energy of 0.4eV to 1.7eV where the direct adsorption took over. The peak area intensity over 1.7eV regions is approximately 1.8 times larger than that for 0.03eV. These results indicate that the incident energy essentially induces the activated adsorption corresponding to the enhancement of the further O₂ dissociative adsorption even at room temperature.