We have developed techniques for high-pressure in situ density measurement of low-Z noncrystalline materials with a diamond-anvil cell (DAC) by an x-ray absorption method. In DAC experiments, accurate determination of the sample thickness is difficult. Moreover, since the sample in a DAC is thin and the interaction between low-Z materials and x rays is small, not the sample but the anvils absorb most of x rays. This makes the measurement quite difficult. We have overcome such difficulties and have successfully measured the density of SiO2 glass, a low-Z noncrystalline material, as a function of pressure up to 35 GPa.