PROBLEM TO BE SOLVED: To provide a method for forming such a thin film of an Sn alloy as to have whisker formation resistance balanced with adequate solderability (low melting point), and have a small and uniform thickness without causing inconvenience such as abrasion due to imperceptible sliding. SOLUTION: The method for forming the thin film of the Sn-Ag-Cu ternary alloy on a substrate comprises immersing the substrate in a plating bath and forming the thin film of the Sn-Ag-Cu ternary alloy on the whole area or a part of the substrate by electroplating, wherein the plating bath includes at least an Sn compound, an Ag compound, a Cu compound, an inorganic chelating agent and an organic chelating agent, wherein the inorganic chelating agent is blended at a ratio of 1 pts.mass to 300 pts.mass with respect to the Ag compound of 1 pts.mass; and the organic chelating agent is blended at a ratio of 1 pts.mass to 200 pts.mass with respect to the Cu compound of 1 pts.mass. COPYRIGHT: (C)2006,JPO&NCIPI