A busbar is placed on an anvil, and a core wire of a cable is placed on the busbar. While the core wire is pressed onto the busbar using a horn chip, ultrasonic vibration is given to the core wire to join the core wire to the busbar. The horn chip has two flat portions and a recessed portion located between the flat portions. When the core wire is pressed onto the busbar using the horn chip, each of the flat portions and the busbar sandwich a part of the core wire therebetween while the recessed portion and the busbar put a remaining part of the core wire therebetween. Each of the sandwiched parts of the core wire does not reach an outer end of the corresponding flat portion to leave a space between the corresponding flat portion and the busbar.