A method of forming an Sn--Ag--Cu ternary alloy thin-film of the present invention forms the ternary alloy thin-film by electroplating. A plating bath contains an Sn compound, an Ag compound, a Cu compound, an inorganic chelating agent and an organic chelating agent. The inorganic chelating agent is one of a polymerized phosphate-based chelating agent and a chelating agent represented by a chemical formula (I): MF.sub.X.sup.(X--Y)-- . . . (I) where M is an arbitrary metal, X is an arbitrary natural number and Y is an oxidation number of M. The organic chelating agent is one of porphyrins, dipivaloylmethane, phthalocyanines and a compound represented by a chemical formula (II): R--(CH.sub.2CH.sub.2O).sub.n-A . . . (II) where R is an alkyl group having a carbon number of 8 to 30, A is CH.sub.2COONa or CH.sub.2SO.sub.4Na and n is a natural number.