In order to clarify the reason why the electrical conductivity of zirconia solid solution decreases abruptly, when it was highly doped with aliovalent oxide, the system (Y1-xScx)(0.3)Zr0.7O1.85 was studied by means of DC- and AC-conductivity measurements. Y0.3Zr0.7O1.85 (x = 0.0) showed lower conductivity than that of Y0.2Zr0.8O1.90. It was found that the decrease in the electrical conductivity was due to the presence of a grain boundary phase as a 'microdomain' by the complex impedance measurements. On the other hand, the substitution of Sc3+ for Y3+ increased the electrical conductivity, and at the same time, the grain boundary phase disappeared. This fact suggested that the high level doping induced a second phase at the grain boundary, resulting in the decrease in the electrical conductivity.