[en] The reliability of the nuclear power stations in Japan is discussed by investigating into availability of the installations. Generally, the expected availability is 70% in view of the periodic inspection and others. Actually, the availability of the total installations fell below 50% in 1975 and 1977. The reasons are the countermeasures to the stress corrosion cracking occurred in the pipings of BWRs, the improvement of water treatment method to prevent the leak in steam generators and so on. The lowest availability occurred in the eighth and ninth reactor years. In Japan, the difference in the highest and lowest availabilities is large as compared with that in the world, showing the inferiority in the operational reliability. But the availability of nuclear power stations is not especially worse than that of the other types of power stations. (Kako, I.)