Abstract: An object-to-be-soldered (92) is accommodated in a sealable chamber (17). An internal pressure (P) of the chamber (17) is raised to a normal pressure (Po) or higher by feeding a reducing gas to the chamber (17). A soldering of a semiconductor element (12) with respect to a circuit board (11) is carried out in the pressurized state. The pressurized state indicating a set pressure (P1) (for example, 0.13 MPa) is maintained in a solder melting period (t3 to t7) until the molten solder (33) is solidified (t7) after the solder (33) starts melting (t3). Accordingly, voids are inhibited from being generated in the solder after being solidified.