To economically advantageously obtain the subject noble metal salt useful for electrolytic metal-plating of silver and silver alloy from electrolyte of noncyanide by reacting a noble metal with an oxidizing reagent in the presence of a non-oxidizing acid.

(A) A noble metal is directly reacted with (B) an oxidizing reagent in the presence of (C) a non-oxidizing acid, preferably at 5-150°C, under 0.1-10 atomic pressure for 1 hr to 21 day to provide the objective noble metal salt. Furthermore, it is preferable that the component A is silver or platinum and the component C is a 1-20C straight-chain alkanesulfonic acid, 3-40C branched chain-like alkanesulfonic acid, especially fluoroboric acid and the component B is preferably hydrogen peroxide or a dialkanesulfonyl peroxide. Specifically, metal silver needle-like crystal having 8-100 mesh size is preferably reacted with a 1-3C straight-chain alkanesulfonic acid at about 2:1 to 1:1 weight ratio of acid to silver in the presence of hydrogen peroxide at about (6:1) to (1-1/2:1) molar ratio of total peroxide to silver.