Here we report the phase boundary between CaCl2-type and α-PbO2-type silica at high pressure and temperature up to 151 GPa and 2500-K determined by in-situ X-ray measurements in a laser-heated diamond anvil cell. Amorphous silica was used as starting material, and the sample was heated for more than 1.5 hr to examine the kinetic effects. The results demonstrated that the CaCl2-type silica is a post-stishovite phase and that it undergoes further transition to the α-PbO2-type structure above 121 GPa at 2400-K. Present data together with previous first-principles calculations indicate that the phase boundary between CaCl2-type and α-PbO2-type silica is represented by a linear equation P (GPa) = 98 + (0.0095 ¿ 0.0016) * T (K). The α-PbO2-type silica can be present in the deep portion of the lower mantle in the silica-saturated bulk compositions. This phase transition might contribute to the seismic wave velocity anomalies observed in the D″ region.