To reduce a software error of a semiconductor device by producing a stable lead having a reduced a ray quantity through an isotope spectrum to produce a lead alloy solder and using as the solder for a semiconductor device.

An isotope is separated by mass spectrum, a stable lead having a reduced α ray quantity only is taken out. A lead is preferably one among 204Pb, 206Pb, 207Pb, 208Pb. A concentration of 210Po in the lead is preferably ≤1 Bq/g. By dissolving the lead (alloy) in an acid and removing 210Bi, 210Po, the stable lead having a reduced α ray quantity is obtained. The lead (alloy) is dissolved in a first acid, this solution is converted to a second acid not dissolving silver or copper, is added with silver or copper and precipitates Po, the lead is taken out from the solution. The first acid is preferably an acid containing a nitric acid, the second acid is preferably a weak hydrochloric acid. Conversion of the second acid preferably uses a sub process to evaporate/ dry/solidify the first acid.