An overlapping portion of an insulating portion formed in an annular shape is held between a workbench having a linear projection and an ultrasonic horn having a circular projection with at least part of the linear projection and of the circular projection opposed to each other and ultrasonically welded. In the ultrasonic welding, even though the ultrasonic horn under ultrasonic vibration rotates on the overlapping portion of the insulating portion, the area in the overlapping portion held between the linear projection and the circular projection with at least part of the linear projection and of the circular projection opposed to each other is not reduced after ultrasonic vibration is applied to the ultrasonic horn.