PURPOSE: To make printing of substrates finer in pitch and to increase the scale of integration of hybrid ICs, etc., by executing reflow in a vessel internally having a reducing gas atmosphere isolated from the outside.

CONSTITUTION: Cream solder is subjected to the reflow in the reducing gas atmosphere in the hermetic vessel 1. The reducing gas mainly consisting of gaseous hydrogen is, thereupon, supplied from a reducing gas supply device 10 into the vessel 1. The cream solder is subjected to the reflow in the vessel 1 internally having the reducing gas atmosphere isolated from the outside. The reducing gas is recovered by a reducing gas recovery device 9 without substantially oxidizing the reducing gas after the reflow and is reutilized for the reflow. As a result, the life of a flux is prolonged and the preservation period of the cream solder is extended. The solder joint parts having the reliability are thus obtd.