The present invention provides a method for conducting electroplating in the presence of CO2 and a metal salt-containing aqueous solution, wherein the CO2 is in the form of liquid, subcritical or supercritical, the method being characterized in that a nonionic compound having a CO2-affinitive moiety is further added to the system where the aqueous solution and CO2 coexist. The method of the present invention improves the efficiency of the electrochemical reaction process and enables formation of an excellent metal film.