A semiconductor device has interconnects protected with an alloy film having a minimum thickness necessary for producing the effect of preventing diffusion of oxygen, copper, etc., formed more uniformly over an entire surface of a substrate with less dependency to the interconnect pattern of the substrate. The semiconductor device includes, embedded interconnects, formed by filling an interconnect material into interconnect recesses formed in an electric insulator on a substrate, and an alloy film, containing 1 to 9 atomic % of tungsten or molybdenum and 3 to 12 atomic % of phosphorus or boron, formed by electroless plating on at least part of the embedded interconnects.