TECHNOLOGY FOCUS - POLYMERS - Preferred Component: The average primary particle diameter of reduced metal particle is 1-100 nm. The surface roughness (Ra) at the side of the reflection layer in the metal-plating undercoat polymer layer is 20 nm or less. The surface resistance value of metal-plating undercoat polymer layer is 0.001-100 Omega /square, preferably 0.03-50 Omega /square. The polymer layer contains acrylic polymer. The acrylic polymer is formed using acrylic polymer having an acidic group and a polymeric group in a side chain. Energy provision is carried out and crosslinked structure or chemical bond is formed between polymeric group in metal-plating undercoat polymer layer and support having polymer layer. The reflectance of mirror film with respect to light having wavelength of 600 nm is 90% or more. Preferred Method: The metal-plating undercoat polymer layer containing the reduced metal particle is formed by forming polymer layer containing metal precursor on support, by providing energy and reducing the metal precursor. The undercoat polymer layer is formed using a copolymer containing a polymerizable group-containing unit of formula (A), an interactive group-containing unit having a non-dissociable functional group of formula (B), and an interactive group-containing unit having an ionic polar group of formula (C). The undercoat polymer layer containing metal precursor is formed on the support using composition for polymer layer containing a material capable of producing active type to support. The unreacted polymer is removed after providing energy with respect to the support having the undercoat polymer layer, and the metal precursor is reduced. The metal precursor is reduced by contacting the metal precursor with aqueous solution containing a reducing agent. The reflection layer containing silver is formed by electroplating and washing the support as electroplating post-process.R1-R6=H or 1-4C optionally substituted alkyl;X,Y',Z,U'=single bond, optionally substituted bivalent organic group, ester, amide or ether;L1-L3=single bond or optionally substituted bivalent organic group;W'=non-dissociative functional group which forms metal-plating catalyst, or its precursor and interaction;andV'=ionic polar group which forms metal-plating catalyst, or its precursor and interaction.