A thermally stimulated depolarization current was measured in pure silica glasses and crystalline quartz, after a linear temperature increase under applied electric field, up to 520 K. The depolarization peak at 480 K disappeared after UV illumination at room temperature. A negative depolarization current was observed in silica glasses after polarization by an electric field under UV illumination at room temperature. There exist two different types of dipoles which are electron-hole pairs, in which electron and hole are trapped on two different defects respectively.