By forming a terminal at a tip of a lead part of a lead frame, and by fixing this terminal and a connecting pad which was formed on an upper surface of a first printed circuit board, the lead frame is attached to the first printed circuit board. By cutting off a frame part and a tie bar part from the lead frame which was attached to the first printed circuit board, the lead part is separated, and forming is applied to the lead part so as for its tip to be extended over the first printed board. After the lead part which is expanded upward is inserted into a through-hole which was opened in a second printed circuit board, by soldering the lead part and the through-hole, the first printed circuit board and the second printed circuit board are electrically connected.