The effects of the addition of Y or both Y and Zr to Bi-Mn-Co-Sb-Si-Cr-Ni-added ZnO varistors (the same composition as one of rcommercial varistors) on the varistor voltage and the tolerance characteristics of electrical degradation were investigated. rThe tolerance characteristic of electrical degradation was deteriorated rapidly by addition of Y of more than approximately 1 rmol%. The degree of deterioration for addition of both Zr and Y was smaller than that for addition of only Y. Moreover, the rnumber of formed spinel (Zn_(2.33)Sb_(0.67)O_4)-type particles monotonically increased with increasing amount of Y. On the rother hand, the number of formed willemite (Zn_2SiO_4)-type particles decreased by addition of Y of more than approximately 1 rmol%. It was suggested that the deterioration of the tolerance characteristics of electrical degradation by the addition of Y ris probably caused by an increase in the number of willemite-type particles. Moreover, it was suggested that the reduction in rthe average ZnO grain size due to the addition of Y was a major factor in the increased varistor voltage and the ZnO grain rgrowth was inhibited by the formation of an unknown compound, of which main ingredients are Y, Zn, and Sb or Y, Zr, Zn, and rSb, after adding Y. The varistor voltage of a varistor with 2 mol% added Y increased by approximately 50% compared to a rvaristor with no Y added. A ZnO varistor with a varistor voltage of approximately 600 V/mm and excellent tolerance rcharacteristics of electrical degradation was fabricated by adding approximately 1.25 mol% Y and approximately 0.45 mol% Zr rto a Bi-Mn-Co-Sb-Si-Cr-Ni-added ZnO varistor.%耐課電劣化特性の良好な酸化亜鉛（ZnO）パリスタにはrn様々な添加物が添加されており，添加物やZnとの間の相互rn作用によって耐課電劣化特性を向上させている。そこrnで，従来報告されている耐課電劣化特性が良好なZnO バリrnスタを作製し，そのバリスタに添加されている添加物の量rnを変化させると，その添加物とほかの添加物の間に働く相rn互作用が変化し，耐課電劣化特性へ影響を与えることが予rn想される。
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