In one embodiment, a first substrate having first solder bumps and a second substrate having second solder bumps are stacked while temporarily tacking the solder bumps to each other, and then a stack is disposed inside a furnace. The gas in the furnace is exhausted to be in a reduced pressure atmosphere, and then a carboxylic acid gas is introduced into the furnace. While increasing a temperature inside the furnace where the carboxylic acid gas is introduced, the gas in the furnace is exhausted to be in a reduced pressure atmosphere at a temperature in a range from a reduction temperature of oxide films by the carboxylic acid gas to lower than a melting temperature of the solder bumps. By increasing the temperature inside the furnace up to a temperature in a range of the melting temperature of the solder bumps and higher, the first solder bumps and the second solder bumps are melted and joined.