In an ultrasonic welding apparatus, ultraviolet-curable ink droplets are discharged to an upper surface of stacked sheets by means of an ink-jet head to land the ink droplets in a predetermined area on the upper surface of the sheets. The sheets, on which the ink droplets have been landed, are interposed and pressed between a welding horn and a receiving stand, while the welding horn is ultrasonically vibrated. Accordingly, the stacked sheets are welded to one another at portions opposed to the predetermined area in which the ink droplets are landed. In this way, the ultrasonic welding apparatus is provided, which makes it possible to perform the ultrasonic welding irrelevant to the shape of the area to be subjected to the welding.