Provided is an ultrasonic horn, the output amplitude of which can be significantly increased. An ultrasonic horn includes an input-side end section that has an input surface that receives ultrasonic vibration, and an output-side end section having an output surface that outputs ultrasonic vibration to a sheet. The output-side end section has a dimension, in the length direction, over which a side edge portion of a front abdominal section and a side edge portion of a rear dorsal section 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 of the output-side end section, positioned on both sides of the slit, functions as the output surface through which the ultrasonic vibration is output.