An Sn alloy plating apparatus is disclosed. The apparatus includes a plating bath configured to store an Sn alloy plating solution therein with an insoluble anode and a substrate immersed in the Sn alloy plating solution, an Sn dissolving having an anion exchange membrane therein which isolates an anode chamber, in which an Sn anode is disposed, and a cathode chamber, in which a cathode is disposed, from each other, a pure water supply structure configured to supply pure water to the anode chamber and the cathode chamber, a methanesulfonic acid solution supply structure configured to supply a methanesulfonic acid solution, containing a methanesulfonic acid, to the anode chamber and the cathode chamber, and an Sn replenisher supply structure configured to supply an Sn replenisher, produced in the anode chamber and containing Sn ions and a methanesulfonic acid, to the plating bath.