The author's aim is to investigate ZnO varistors with high voltage to 1000V/mm and good resistance to electrical degradation, which could be applied to the UHV transmission systems and reduce ZnO varistor element amounts in surge arresters. The conduction processes in ZnO varistors is clarified. By adjusting additive type and amounts in ZnO the electrical properties of ZnO varistors can be controlled. Based on this recipe, Y, Cr, Ni, B, Si oxides were added in this study. These oxides can be divided into two categories, the Y doping group and the Si doping group. / 博士(工学) / Doctor of Philosophy in Engineering / 同志社大学 / Doshisha University