Abstract: In this invention, the rate of decrease of an oxidation suppressing element in a solder bath during use is measured, and a solder alloy including the oxidation suppressing element in the same or greater proportion than the rate of decrease in the amount is suitably added in accordance with the decrease in the solder bath. As a simple method, in flow soldering of an Sn—Ag based or Sn—Ag—Cu based solder alloy, to compensate for a decrease in the P content of the solder bath which is observed during operation, a solder alloy containing 60-100 ppm by mass of P in an Sn—Ag based or Sn—Ag—Cu based solder alloy for replenishing a solder bath is supplied to maintain not only the P content but also the surface level of molder solder bath.