In electric plating of supplying a current between an anode electrode and a cathode electrode as a plated body immersed in a plating solution thereby forming a plated film comprising a conductor on a surface of the cathode electrode, a preliminary electrolytic electrode that comes in contact with the plating solution before the cathode electrode comes in contact with the plating solution is disposed, and the cathode electrode is brought into contact with the plating solution while supplying a preliminary electrolytic current between the preliminary electrolytic electrode and the anode electrode, whereby a uniform plated film with no voids can be formed while suppressing dissolution of the underlying conductive film in the electric plating treatment.