Abstract: In interconnecting printed circuit boards: preparing a first and second printed circuit board is accomplished with the first having an insulating substrate of thermoplastic resin and a conductive pattern with a land, while the second has a conductive pattern with a land; overlapping the land of the first with the land of the second is done to form an interconnection portion; and heating the interconnection portion at a temperature approximately higher than a glass transition temperature of the thermoplastic resin while applying pressure to the interconnection portion to create an electrical interconnection sealed with a part of the thermoplastic resin constituting the insulating substrate of the first board is accomplished. The insulating substrate of the first board is overlapped with an insulating substrate of the second printed board to interpose a film, the film including material to reduce a modulus of elasticity of the insulating substrate of the first board.