A board piece 2 of the present invention comprises a non-thermoplastic resin film 11, a thermoplastic resin film 10 formed on the non-thermoplastic resin film 11 and a metal wiring 8 formed on the surface of the thermoplastic resin film 10. Metal wiring 8 is partially exposed on board piece 2 to form a contact 12. A low-melting metal coating 13 is formed on contact 12 and two board pieces 2a, 2b are pressed against each other under heating with contacts 12a, 12b thereof being in contact with each other so that thermoplastic resin films 10a, 10b soften to adhere board pieces 2a, 2b to each other and low-melting metal coatings 13a, 13b melt and then solidify to connect contacts 12a, 12b to each other. The region of metal wiring 8 not used for connection is wiring 17 connecting contacts 12 to each other and a cover film 19 can be provided on the surface thereof. Contacts 12a, 12b can also be connected by applying ultrasonic wave.