A method of forming Cu plating of the present invention includes: a first step of forming a Cu seed layer on one of surfaces of a substrate such that an average grain size is 50 nm or more and 300 nm or less; a second step of forming an oxide film on a surface of the Cu seed layer in an oxygen atmosphere; a third step of removing a part of the oxide film; and a fourth step of feeding power to the Cu seed layer to form Cu plating on a surface of the oxide film on the Cu seed layer by electrolytic plating.