To prevent the occurrence of whisker by a simple method capable of easily controlling the formation of a tin plating film formed on an outer lead of a lead frame under a conventional plating condition without deteriorating the productivity.

A tin-silver coating film in which the content of silver is 0.4-3.5 wt.% is formed by forming a tin plating film having 8-20 m thickness on a material to be plated such as the lead frame and depositing silver on the tin plating film and thermally diffusing as a post treatment of the tin plating. The deposition of silver is carried out by a simple method of only dipping into a treating solution.