In an Sn-plated strip in which a copper alloy containing 15 to 40 mass % of Zn in terms of an average concentration is used as an alloy strip and the layers of an Sn phase, an Sn—Cu alloy phase and an Ni phase constitute a plating film from the surface to the alloy strip, the Zn concentration of the surface of the Sn phase is adjusted to a range of 0.1 to 5.0 mass %. The alloy may further contain 0.005 to 3.0 mass % in total of an arbitrary constituent selected from Sn, Ag, Pb, Fe, Ni, Mn, Si, Al and Ti. Moreover, the alloy may be a copper base alloy containing 15 to 40 mass % of Zn, 8 to 20 mass % of Ni, 0 to 0.5 mass % of Mn and a balance of Cu and unavoidable impurities, and may further contain 0.005 to 10 mass % in total of the above arbitrary constituent. There is provided a Cu/Ni double layer base reflowed Sn-plated Cu—Zn alloy strip in which generation of whiskers is suppressed.