A method includes: generating a peroxide radical on a dielectric substrate surface by treating the dielectric substrate surface with atmospheric pressure plasma using a rare gas; fixing a functional group forming a coordinate bond with a silver ion, by reacting a grafting agent; and applying a silver-containing composition to the substrate surface, followed by heating and curing the silver-containing composition, to thereby form a silver thin film layer, the silver-containing composition containing a silver compound (A) represented by Formula (1) and an amine compound (B) represented by Formula (2), the silver compound (A) being contained in an amount of 10 to 50% by mass, the amine compound (B) being contained in an amount of 50 to 90% by mass, relative to a total amount of 100% by mass of the silver compound (A) and the amine compound (B). The method enables to form a metal film having high adhesiveness even on the surface of a fluorine resin, which is suitable as a dielectric substrate due to its property of avoidance of delay in signal transmission speed or increase in power consumption, but has extremely low adhesiveness. [C. 1]


(R 1 : a hydrogen atom, -(CY 2 )a-CH a , or -((CH 2 )b-O-CHZ)c-CH 3 ; R 2 : -(CY 2 )d-CH 3 or -((CH 2 )e-O-CHZ)f-CH 3 ; Y: a hydrogen atom or -(CH 2 )g-CH 3 ; Z: a hydrogen atom or -(CH 2 )h-CH 3 ; a: an integer of 0 to 8; b: an integer of 1 to 4; c: an integer of 1 to 3; d: an integer of 1 to 8; e: an integer of 1 to 4; f: an integer of 1 to 3; g: an integer of 1 to 3; h: an integer of 1 or 2)