The present invention provides a processing method capable of continuously working an Au—Sn soldering metal having a foil form in room temperature. The foil-form soldering metal containing from 10% by weight to 90% by weight of Au and balance comprising Sn is subjected to heat treatment for five minutes to ten hours at 200 C. to 270 C., and subsequently the foil-form soldering metal is slit. Thus, the heat treatment of the Au—Sn soldering metal before slitting enables continuous slitting of the Au—Sn foil-form soldering metal in room temperature and facilitates the production of a ribbon-form soldering metal.