PURPOSE: To increase the void elimination effect within solder in reflow soldering.

CONSTITUTION: A component assembly body where solder is included between components is supplied to a reflow oven 1 and a preheating process 1a, a solder melting process 1b, and a cooling process 1c are performed successively, and pressure applied to the component assembly body at the solder melting process 1b is changed, thus causing the void in the solder to be pushed out of the component assembly easily. The component assembly body is positioned at an auxiliary tool 2. In the component assembly auxiliary tool 2, a press part for applying pressure to the component assembly body is provided. The press part can be displaced freely in the direction of press and the press force can be changed according to the displacement. Also, a pressure reception part 13 which is linked to the press part is provided. In the reflow oven 1, a guide surface 15 where the pressure reception part 13 of the component assembly auxiliary tool 2 is provided corresponding to the part for performing the solder melting process 1b. When the pressure reception part 13 abuts on the guide surface 15, pressure applied to the component assembly body changes.