A metal joining method comprising the steps of forming a joining layer including metal nanoparticles between the surface metals of two joined members and heating it to join the surface metals of the two joined members, wherein said joining layer is comprised of a first joining layer containing metal nanoparticles, a metal foil, and a second joining layer containing metal nanoparticles in this order, and wherein said metal foil is Al, an Al alloy, Ag, an Ag alloy, Au or an Au alloy, and, when said metal foil is Al or an Al alloy, said metal nanoparticles are Ni nanoparticles, and when said metal foil is any one of Ag, an Ag alloy, Au or an Au alloy, said metal nanoparticles are nanoparticles comprised of at least one of Ag, Au, Cu, or Ni, and the heating temperature is 200° C. or more,
- wherein Al content of the Al alloy is 85 mass % or more.