A method for producing a nano-composite metal member, by which a nano-composite metal member can be readily produced and the production cost can be reduced, and a method for joining phase-separated metal solids using the principle same as that of the former method are provided. A nano-composite metal member is obtained by bringing a solid metal body comprising a first component into contact with a solid metal material comprising a compound, an alloy or a non-equilibrium alloy that simultaneously contains a second component and a third component having a positive heat of mixing and a negative heat of mixing, respectively, relative to the first component, and then performing heat treatment at a predetermined temperature for a predetermined length of time, so as to cause interdiffusion between the first component and the third component.