A model explaining the formation of electric domain structure in heavily doped ruby under intensive laser irradiation is suggested. The model is based on the following facts: thermalization of metastable level E of Cr³⁺; distribution in the A and B sublattices during crystal growth hopping conductivity in a system of Cr³⁺ ions excited into a metastable level. It is demonstrated experimentally that the number of Cr³⁺ ions decreases during the formation of internal photoelectrical structure in ruby, the distribution of Cr³⁺ ions in the A and B sublattices is inhomogeneous in the sample, and the rate of formation of the domain structure depends on the irradiation prehistory of the sample. Observing the dynamics of the electric domain structure in ruby, conclusions on the microscopic nature of the formation of internal electric fields and the close arrangement of the excess charged chromium ions in domain walls are drawn. The suggested thermalizational model describes a starting mechanism for the formation of internal electric fields in domains and explains qualitatively most of the known experimental results.