A nozzle repair method and a nuclear reactor vessel include: removing a trepanning portion (208) as a connection portion with respect to an in-core instrumentation cylinder (204) in a groove-welding portion (206); removing the in-core instrumentation cylinder (204) from a semi-spherical portion (66) as a lower end plate; forming a surface buttered-welding portion (210) by buttered-welding the surface of the groove-welding portion (206); forming a welding groove (212) by grooving the surface buttered-welding portion (210); inserting a new in-core instrumentation cylinder (204A) provided with a circumferential groove portion (204f) outside an instrumentation equipment guide passage (204d) into an attachment hole (203); and fixing the new in-core instrumentation cylinder (204A) by groove-welding the welding groove (212). Accordingly, since the nozzle welding area is suppressed to a predetermined range, the workability of the repair is improved.