A Cu wiring forming method of forming Cu wiring that is to be arranged in contact with tungsten wiring, by filling Cu into a recess formed in a substrate, includes: removing a tungsten oxide formed on a surface of the tungsten wiring; forming a nitriding preventing film at least on the surface of the tungsten wiring in the recess; forming a barrier film that prevents diffusion of Cu, on a surface in the recess from above the nitriding preventing film; forming a liner film on the barrier film; and filling a Cu film on the liner film.