Provided is an ultrasonic horn, the output amplitude of which can be significantly increased. An ultrasonic horn (9) includes an input-side end section (9a) that has an input surface (9c) that receives ultrasonic vibration, and an output-side end section (9b) having an output surface (9e) that outputs ultrasonic vibration to a sheet (W). The output-side end section (9b) has a dimension (D3), in the length direction, over which a side edge portion of a front abdominal section (20a) and a side edge portion of a rear dorsal section (20b) can be welded in the longitudinal direction, and is branched in a width direction that is perpendicular to the length direction by a slit that extends in the length direction. A pair of surfaces (9e) of the output-side end section (9b), positioned on both sides of the slit (9d), functions as the output surface through which the ultrasonic vibration is output.
