In a Cu wiring structure forming method, a barrier film serving as a Cu diffusion barrier is formed at least on a surface of a recess in a first insulating film formed on a substrate, and the recess is filled with an Al-containing Cu film. A Cu wiring is formed from the Al-containing Cu film, and a cap layer including a Ru film is formed on the Cu wiring. Further, an interface layer containing a Ru—Al alloy is formed at an interface between the Cu wiring and the cap layer by heat generated in forming the cap layer or by a heat treatment performed after forming the cap layer. A second insulating film is formed on the cap layer.