An ultrasonic processing system and an ultrasonic processing method for obtaining seams of preferred finish. An ultrasonic processing system (50) adapted to subject a web (31) running in a machine direction (MD) to ultrasonic processing includes a first mechanical element defined by one of an ultrasonic horn (67) and an anvil and a second mechanical element defined by a remainder. The web (31) placed on an outer peripheral surface (63) of a rotary drum (62) moves conforming to the drum (62) in a machine direction (MD). The first mechanical element is provided on the inner side of the drum (62) and the second mechanical element is provided on the outer side of the drum (62). The first and second mechanical elements repeat a step of forward movement and a step of backward movement in a direction intersecting the machine direction (MD) across the web (31). In both steps, the first and second mechanical elements come in close contact with each other by the intermediary of the web (31) and thereby subject the web (31) to the ultrasonic processing.
