The effect of heat treatment on electrical properties of a quartz glass prepared by flame fusion was investigated and the following results were obtained. (1) When the glass was heated at 1000°C, the electrical conductivity increased irreversibly with time at first and then decreased. (2) The increase in conductivity went on uniformly in the interior of the glass. On the other hand, the decrease in the conductivity started at the surface layer of the glass and spread to the interior. (3) The glass approached finally a state in which the electrical conductivity changed reversibly between high and low conductive states depending on the secondary heat treatment conditions. Such irreversible and reversible changes can be explained by considering the chemical equilibrium between OH group and molecular water in silica glass, outward diffusion of molecular water and protonic conduction.