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 valve body defines a flow passage. The intermediate portion includes an arcuate outer surface section that has teeth to be releasably engaged with the retaining mechanism, allowing the head to be pivotally moved to a desired position relative to the handle and retaining the head in the desired position.