After treated in a solution containing ozone, a plating material is brought into contact with a solution containing at least one of an anionic surface active agent and a nonionic surface active agent, and an alkaline component. Ozone acts to locally break unsaturated bonds on a surface of the plating material to form C—OH bonds or C═O bonds, thereby activating the surface of the plating material, and since a surface active agent 1 is adsorbed thereon, a catalyst 2 is adsorbed on hydrophilic groups of the surface active agent 1 which has been adsorbed on the above-described functional groups. Consequently, no etching treatment is required, and an electroless plated coating having excellent adhesion can be formed without roughening the surface of the resin material.