Abstract: The present invention provides a method for forming a protective film selectively on metal interconnects, such as copper interconnects, of a substrate having an embedded interconnect structure, without causing the problem of contamination of the interconnects with an-alkali metal. The method for forming a protective film according to the present invention comprises: providing a substrate having embedded interconnects formed in a surface of the substrate; and bringing the surface of the substrate into contact with an electroless plating bath, thereby forming a protective film having a film thickness of 0.1 to 500 nm selectively on the exposed surface of the embedded interconnects; wherein the electroless plating bath contains cobalt ions, phosphinate ions and a complexing agent, uses cobalt phosphinate as a main supply source of the cobalt ions and the phosphinate ions, and does not substantially contain alkali metal ions.