Ultrasonic vibration is outputted from an output surface (332) of an ultrasonic horn (330) to a predetermined region (60) of each of a first web (10A) and a second web (20A) to join the predetermined regions (60) together. The output surface (332) is provided with a center output section (332A) which is located at the center region (60C) of the three regions formed by equally dividing the predetermined region (60) into three, and also with opposite-end output sections (332B) which are located at the regions (60S) on both sides of the center region (60C). The maximum amplitude of the ultrasonic vibration at the center outputted from the center output section (332A) is smaller than the maximum amplitude of ultrasonic vibration at opposite ends outputted from the opposite-end output sections (332B). The joint strength at the center region (60C) is lower than the joint strength at the regions (60S).
