Recent advances in nuclear-fusion-reactor development renewed interest in the well-known phenomenon called radiation-induced conductivity (RIC). RIC involves an increase of electrical conductivity of ceramic insulators under exposure to ionizing irradiation. We tried to measure RIC of single crystal a-alumina in the fission reactor JMTR. The a-alumina was encapsulated in a well electrically insulated casing and was exposed to fast neutron irradiation up to 3.4 × 10¹⁷ n/m² s and to γ-ray irradiation up to 1.5 × 10⁴ Gy/s. The measured RIC was 0.6 to 1.5 × 10^-5 (ohm m)^-1 at the maximum exposure rate. Temperature dependence of the RIC was not observed in the present experiment, and the RIC was found to depend on the 0.75th power of reactor power.