The present invention relates to: a washing solution for a tin plating film, which can be used for washing in advance of the washing with water after electroless tin plating; and a method for forming a tin plating layer, which includes a step of washing with the washing solution. The washing solution according to the present invention is an acidic aqueous solution containing an acid, a complexing agent, a stabilizer and a chloride ion. The washing solution contains the chloride ion at a concentration of 2% by weight or more, and also contains tin at a concentration of 0.5% by weight or less. The washing solution according to the present invention has good washing performance against the surface of a tin plating film, can maintain the properties of the tin plating film readily, has a small influence on the surface of the plating film even when used continuously, and has excellent long-term stability.
