A method for producing a soldered product, comprising: a provision step of providing a solder and a temporary fixing agent for temporarily fixing the solder; a temporary fixing step of temporarily fixing the solder to a soldering target with the temporary fixing agent; a vaporization step of placing the soldering target with the solder temporarily fixed thereto in a vacuum or heating the soldering target with the solder temporarily fixed thereto to a predetermined temperature lower than the melting temperature of the solder, to vaporize the temporary fixing agent in order to form gaps between the solder and the soldering target; a reduction step, performed concurrently with or after the vaporization step, of reducing, with a reducing gas at a predetermined temperature lower than the melting temperature of the solder, the solder and the soldering target left in the vaporization step; and a solder melting step, performed after the reduction step, of heating the soldering target to a predetermined temperature equal to or higher than the melting temperature of the solder to melt the solder; and further comprising: an inert gas step, performed before the vaporization step, of placing the soldering target with the solder temporarily fixed thereto in an inert gas; and wherein, in the solder melting step, the solder is melted by heating the soldering target to a temperature equal to or higher than the melting temperature of the solder while the soldering target is in the vacuum; and the method further comprising: a vacuum breaking step, performed after the solder melting step, of increasing pressure to break the vacuum in order to compress or eliminate cavities or voids in the solder, wherein the solder is in a molten state in the vacuum breaking step; and a cooling step, performed after the vacuum breaking step, of cooling the soldering target, wherein the molten solder is cooled and solidified. a provision step of providing a solder and a temporary fixing agent for temporarily fixing the solder; a temporary fixing step of temporarily fixing the solder to a soldering target with the temporary fixing agent; a vaporization step of placing the soldering target with the solder temporarily fixed thereto in a vacuum or heating the soldering target with the solder temporarily fixed thereto to a predetermined temperature lower than the melting temperature of the solder, to vaporize the temporary fixing agent in order to form gaps between the solder and the soldering target; a reduction step, performed concurrently with or after the vaporization step, of reducing, with a reducing gas at a predetermined temperature lower than the melting temperature of the solder, the solder and the soldering target left in the vaporization step; and a solder melting step, performed after the reduction step, of heating the soldering target to a predetermined temperature equal to or higher than the melting temperature of the solder to melt the solder; and further comprising: an inert gas step, performed before the vaporization step, of placing the soldering target with the solder temporarily fixed thereto in an inert gas; and wherein, in the solder melting step, the solder is melted by heating the soldering target to a temperature equal to or higher than the melting temperature of the solder while the soldering target is in the vacuum; and the method further comprising: a vacuum breaking step, performed after the solder melting step, of increasing pressure to break the vacuum in order to compress or eliminate cavities or voids in the solder, wherein the solder is in a molten state in the vacuum breaking step; and a cooling step, performed after the vacuum breaking step, of cooling the soldering target, wherein the molten solder is cooled and solidified. an inert gas step, performed before the vaporization step, of placing the soldering target with the solder temporarily fixed thereto in an inert gas; and wherein, in the solder melting step, the solder is melted by heating the soldering target to a temperature equal to or higher than the melting temperature of the solder while the soldering target is in the vacuum; and the method further comprising: a vacuum breaking step, performed after the solder melting step, of increasing pressure to break the vacuum in order to compress or eliminate cavities or voids in the solder, wherein the solder is in a molten state in the vacuum breaking step; and a cooling step, performed after the vacuum breaking step, of cooling the soldering target, wherein the molten solder is cooled and solidified. a vacuum breaking step, performed after the solder melting step, of increasing pressure to break the vacuum in order to compress or eliminate cavities or voids in the solder, wherein the solder is in a molten state in the vacuum breaking step; and a cooling step, performed after the vacuum breaking step, of cooling the soldering target, wherein the molten solder is cooled and solidified.