Abstract: A Ni-base alloy large-size member comprises a base material having strip-shaped carbide segregation, and a homogeneous modified layer formed on a welding groove surface by treatment using Friction stir processing and solution treatment. In addition, a Ni-base alloy welded structure is manufactured by welding the Ni-base alloy large-size member with other member constituting the Ni-base alloy welded structure, such as a member formed of ferrite steel or another Ni-base alloy large-size member under the condition that weld penetration depth is shallower than the thickness of the modified layer.