Integrity assessment was performed for the reactor vessel of Mihama-1, which started operation early among Japan's PWRs, in view of neutron irradiation Bembrittlement to be undergone after 60 years of operation. Reference temperature RT_NDT and upper shelf energy after 60 years of operation was estimated based on the results of surveillance tests. It was confirmed that the fracture toughness evaluation based on the surveillance tests and speculated reference temperature exceeded the stress intensity factor K₁ which occurred in the event of pressurized thermal shock, and the decreased upper shelf energy evaluated in accordance with the elasto-plastic fracture mechanics would not lead to the growth of an assumed crack followed by fracture after 60 years of operation. The results of evaluations have demonstrated that the integrity of the Mihama unit No. 1 reactor vessel could be maintained after 60 years of operation.