PURPOSE: To prevent the generation of erupting phenomenon of a solder and to prevent sticking of a solder on the peripheral part of a solder bath.

CONSTITUTION: The solders 4 in an upper chamber 6 and a lower chamber 7 of the solder bath 1 are simultaneously heated by upper heaters 2 and lower heaters 3 in the upper and lower chambers 6 and 7 respectively. When the temperature in the lower chamber 7 is detected to be 180°C lower than the melting point of the solder 4, the lower heaters 3 are controlled by a temperature adjusting mechanism to maintain the temperature of 180°C in the lower chamber 7 and heating of the upper chamber 6 by the heaters 2 is continued. Then, when the solder 4 in the upper chamber 6 is heated to 190°C higher than its melting point, the lower chamber 7 is again heated by the heaters 3 to melt the solder 4 in it.