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 α-alumina in the fission reactor JMTR. The α-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.