There are lots of ions Cr3+ in dichromated gelatin (DCG) after exposure and processing. Since the density distribution of Cr3+ is proportional to the exposure of light, distribution of Cr3+ will be sinusoidal. Meanwhile, the ions or molecules of other materials are distributed uniformly in a certain range around Cr3+. Therefore, the distributions of positive and negative charges will be unequal, and an electric field will be induced inside the medium of volume hologram in macro-scale. The intensity of such electric field is calculated basing on this mechanism, and it is found to be considerably strong.