Abstract: An electrolytic plating method, including: preparing a plating solution including water which is a primary medium, a sulfonic acid, and tin, copper, and silver ions, and a complexing agent, concentrations of the sulfonic acid, and tin, copper, and silver ions being 0.5 to 5 mol/L, 0.21 to 2 mol/L, 0.002 to 0.02 mol/L, and 0.01 to 0.1 mol/L, respectively, a mole ratio of the silver-ion concentration to the copper-ion concentration being in a range of 4.5 to 5.58; and separating a solution around a soluble anode containing 90 percent or more of tin from the plating solution on a cathode side by a non-ionic micro-porous membrane having a pore diameter of 0.01 to 0.05 ?m and a thickness of 5 to 100 ?m.