To prevent failures in bonding associated with defective heating of any parts in soldering two parts of different heat capacity.

A land 2 and a terminal 3 are soldered after the step in which the terminal 3 of large heat capacity is independently heated by a soldering iron 5, and the step in which the land 2 whose heat capacity is smaller than that of the terminal 3 is simultaneously heated by the soldering iron together with the terminal 3 to set the land 2 and the terminal 3 at appropriate temperature.