The electrical conductivity of two ceramic insulators, alumina, and silicon nitride was measured in a fission reactor under a dynamic vacuum condition. The instrumented irradiation rig was dynamically evacuated during fission reactor operation. The uppermost vacuum level attained was better than 10-2 Torr. In this experiment, we attempted to reveal effects of the gaseous environment on the measurement of electrical conductivity under ionizing irradiation. The results showed that the gaseous environment has a most hazardous effect in a certain gas pressure range.