A method for electrically connecting an upper wiring with a lower wiring consists of the steps of arranging the upper wiring mixed with thermo-softening material on an upper insulating sheet, arranging the lower wiring mixed with thermo-softening material on a lower insulating sheet, putting the upper insulating sheet on the lower insulating sheet to bring the upper wiring into contact with the lower wiring, and providing ultrasonic vibration to the upper and lower wirings and the upper insulating sheet while pushing the upper insulating sheet toward the lower insulating sheet. In these steps, the upper and lower wirings and the upper insulating sheet are heated and melted because friction heat is generated by the ultrasonic vibration. Thereafter, the upper insulating sheet is pushed into the upper and lower wirings while pushing these wirings aside and is boded with the lower insulating sheet. Also, the upper wiring is bonded with the lower wiring so that these wirings are electrically connected.