The compression behavior of fully densified SiO2 glass has been measured up to 9 GPa at room temperature by using a diamond-anvil cell with a mixture of methanol-ethanol as a pressure medium. Optical-microscope observations clarify that there is remarkable agreement between the volume data on compression and decompression and therefore the glass behaves in an elastic manner. The compression curve can be expressed accurately enough by a Birch-Murnaghan equation of state with the zero-pressure bulk modulus K-0 = 60.2 GPa and its pressure derivative K'(0) = 4 (fixed). Extrapolating this equation of state to higher pressures, the density of the glass merges with that of ordinary glass (i.e., former ordinary glass compressed to high pressures) at about 13 GPa. X-ray diffraction and Raman scattering measurements show that the first sharp diffraction peak and the main Raman band of the glass also merge with those of ordinary glass at the similar pressure range. These results suggest that the compaction of interstitial voids dominates in compression mechanisms of densified SiO2 glass, similar to the case for ordinary SiO2 glass. Together with available information from the literature, it is presumed that the permanent densification of SiO2 glass takes place between 9 and 13 GPa at room temperature. The behavior of SiO2 glass in its intermediately densified states is also discussed.