PURPOSE: To manufacture a varisotr having less irregularity in a stable manner by adjusting the atmosphere of a diffusion heat treatment.

CONSTITUTION: In order to enhance the crystal grain boundary of strontium titanate varistor in resistance a diffusion heat treatment is conducted on a crystal grain boundary using a diffusion agent. The surface of the sintered body obtained by firing is coated with sodium carbonate of 5wt.% against the sintered body of 100wt.% as a diffusion agent. A diffusion heat treatment is conducted for one hour using nitrogen-oxygen mixed gas of total pressure 2atm in the atmosphere of partial pressure of oxygen or total pressure of oxygen of 1.1 to 1.8atm. The temperature of the diffusion heat treatment is used to obtain the current-voltage characteristics of V1mA=240V.