PURPOSE: To eliminate positional displacement of two substrates by relatively securing first and second substrates by a temporarily clamping solder layer during soldering by a paper reflowing method.

CONSTITUTION: Solder layers 513, 514 are melted by irradiating them with a laser beam. When the irradiation of the beam is stopped, the solder of melted state is solidified by natural cooling, and substrates 1, 2 are bonded to be temporarily clamped by the layers 513, 514. This is filled in a paper reflowing tank to melt other solder layers 511, 512 by a paper reflowing method to solder the substrate 1 to the substrate 2. Since the melting point of the solder of the layers 513, 514 is so set as to be higher than that of the solder of the other layer 511, 512, the solders of the layers 513, 514 are not melted. Thus, positional displacement of the thus connected substrates is prevented.