This paper deals with the theoretical analysis and experimental investigation of a new optical DC voltage measuring system, in which two Pockels cells are located between a polarizer and an analyzer, and AC voltage is applied to one cell while DC voltage for measuring is applied to the other. Supposing a slight elliptical polarization occurred at no applied voltage when linearly polarized light passed through the Pockels cells without optical activity, the theoretical analysis of the measuring system was carried out. It clarified that the light intensity variation in the system could be compensated by dividing the AC normal component V(ω₀) of the output by the second harmonic component V(2ω₀), and V(ω₀)/V(2ω₀) was linearly proportional to the applied DC voltage, but it was not equalto zero at no applied DC voltage. Experiments were carried out by using Bi₄Ge₃O₁₂ (BGO) crystals longitudinally modulated with AC and DC voltage and showed that the characteristics of the DC component, V(ω₀), and V(2ω₀) of the output agreed well with the calculated results. Slightly rotating the y-z surface of the BGO about x axis, it was clarified that the phase difference caused by spontaneous birefringence in the BGO could be decreased to zero and linear DC voltage dependence of V(ω₀)/V(2ω₀) passed through the origin could be obtained at no apparent elliptical polarization.