A solution treatment is firstly performed for a non-heat-treated Ni based alloy having a composition equivalent to that of Inconel 718 (registered trademark). Subsequently, a primary aging treatment is applied by holding the Ni based alloy at 610 to 660 C. for 5 to 10 hours. After that, a secondary aging treatment is performed by holding the Ni based alloy at 710 to 760 C. for 5 to 10 hours. There are 700 or more precipitates per m², in which each precipitate has a longer diameter of not less than 0.5 nm, in a metal microstructure of the Ni based alloy. Some of the precipitates are large precipitates having average diameters of 25 nm to 1 m. There are 10 or more large precipitates per m². A forging die is produced with the Ni based alloy.