Provided is a metal double-layer structure in which a modified metallic member modified from a flat plate metallic member is bonded to be stuck to a plate material, manufacturing method thereof, and a method of regenerating a sputtering target using the method. The method includes the steps of: overlapping the plate material with the metallic member; inserting a rotary tool having a rotor and a probe projecting from a bottom surface of the rotor into a surface of the metallic member while rotated; bringing a distal end of the probe to a position close to a mating plane between the metallic member and the plate material to generate friction heat and stir the distal end, and moving the rotary tool to form adjacent motion tracks on the surface of the metallic member; and forming stirred areas along the mating plane to bond the metallic member and the plate material together, and modifying the metallic member into a modified metallic member.