To provide a method for Sn alloy electrolytic plating that allows use of a soluble anode by resolving the problem of metal deposition on an anode when a Sn alloy, such as a Sn-Ag-based alloy, is subjected to electrolytic plating.

The interior of a plating tank 1 is divided into a cathode chamber 4 and an anode chamber 3 by an anion-exchange membrane 2. A Sn ion-containing plating solution is supplied to the cathode chamber 4, and an acid solution is supplied to the anode chamber 3. While electrolytic plating is performed by applying an electrical current between an object to be plated 12 in the cathode chamber 4 and a Sn anode 11 in the anode chamber 3, the acid solution containing Sn ions eluted from the Sn anode 11 as plating progresses is used as a Sn-ion resupply liquid for the plating solution in the cathode chamber 4.