A first printed circuit board includes a first insulating board substantially made of thermoplastic resin and a first conductive pattern. A second printed circuit board includes a second insulating board and a second conductive pattern. The first and second printed circuit boards are overlapped. The overlapped portion of the first and second printed circuit boards is heat pressed to connect the first and second printed circuit boards with a heat-pressing tool while the first insulating board is cooled at an area adjacent to an area pressed by the heat-pressing tool. With the heat pressing, the first and second lands are electrically connected and the thermoplastic resin is softened and plastically deformed to bond the first insulating board to the second insulating board. The thickness of the first insulating board is preferably controlled at the heat pressed area and the area adjacent to the heat pressed area by the cooling.