In a ultrasonic processing apparatus, upstream conveyor and downstream conveyor allow a fibrous web to run in a machine direction continuously. Between the conveyors, a working surface of a first mechanical element serving as a stationary element of the ultrasonic processing apparatus extends diagonally with respect to the machine direction. The fibrous web runs in the machine direction above the working surface. The ultrasonic processing apparatus includes a second mechanical element adapted to move in a direction diagonally intersecting the machine direction. The working surface of the first mechanical element cooperates with the second mechanical element to subject the fibrous web to ultrasonic processing. One of an ultrasonic horn and an anvil is used as the first mechanical element and a remainder of the ultrasonic horn and the anvil is used as the second mechanical element.