[en] For the purpose of developing a proper testing method to evaluate the radiation resistivity of the insulation materials for electrical cables used for nuclear reactors, the investigation of radiation oxidation aging in which the materials to be tested are irradiated under oxygen pressurization has been continued, to estimate the aging of insulation materials in the normal operation of a reactor (50 to 60 deg C at low dose rate), and to find the accelerated aging method equivalent to this condition. On the basis of these basic experimental results, a testing facility has been produced for trial, and installed, which degrades electrical cables (600V, 3C x 5.5 mm²) by oxidation under the exposure to the radiation of rather high dose rate (0.26 Mrad/h). It has been clarified after the experiment for one year that the facility realized the expected performance, and it was safe without any problem. It was found that the aging of three types of cables, CV, PN, PH, (CV: cross-linked polyethylene-insulated, PVC-sheathed cable; PN: ethylene-propylene rubber-insulated, Neoprene-sheathed cable; PH: ethylene-propylene rubber-insulated, Hypalon-sheathed cable) by irradiation under oxygen pressurization was nearly equal to the aging by long term irradiation in air at low dose rate (5 krad/hr). Thus, the above described method is promising to be able to simulate the low dose rate aging. As for the complex degradation in which thermal aging was added to the above, the thermal aging after irradiation was accelerated by oxidation under the application of radiation. This seems to be the major cause of multiplying effect in complex thermal and radiation aging. (Wakatsuki, Y.)