An apparatus for repetitive ultrasonic processing of a fibrous web continuously running in a machine direction by first and second mechanical elements (67, 68) adapted to sandwich the fibrous web in its thickness direction, wherein the apparatus comprises: an upstream side conveying means and a downstream side conveying means adapted to continuously run the fibrous web in the machine direction; a drum located between the upstream side conveying means and the downstream side conveying means, the drum having an outer peripheral surface adapted to continuously rotate in one direction at a circumferential velocity equal to a running velocity of the fibrous web and adapted to support the fibrous web placed thereon; characterised by: the outer peripheral surface being formed with at least one ultrasonic processing space being in communication with the inside and the outside of the drum; the first mechanical elements (67, 68) each being located inside the drum in one of the processing spaces so that the first mechanical element is adapted to repetitively move back-and-forth in a direction crossing the machine direction to pass transversely across the fibrous web; the second mechanical elements (67, 68) each being located outside the drum in one of the processing spaces and adapted so that the second mechanical element (67, 68) repetitively moves back-and-forth in synchronization with the back-and-forth movement of the first mechanical element (67, 68) in the same direction as the direction in which the first mechanical element (67, 68) moves back-and-forth; and the first and second mechanical elements being adapted to cooperate with each other to subject the fibrous web lying in each of the processing spaces during one of the forward and backward movements to ultrasonic processing, and at least one of the first and second mechanical elements is adapted to be spaced from the fibrous web in order to stop the ultrasonic processing during the other of the forward and backward movements. an upstream side conveying means and a downstream side conveying means adapted to continuously run the fibrous web in the machine direction; a drum located between the upstream side conveying means and the downstream side conveying means, the drum having an outer peripheral surface adapted to continuously rotate in one direction at a circumferential velocity equal to a running velocity of the fibrous web and adapted to support the fibrous web placed thereon; characterised by: the outer peripheral surface being formed with at least one ultrasonic processing space being in communication with the inside and the outside of the drum; the first mechanical elements (67, 68) each being located inside the drum in one of the processing spaces so that the first mechanical element is adapted to repetitively move back-and-forth in a direction crossing the machine direction to pass transversely across the fibrous web; the second mechanical elements (67, 68) each being located outside the drum in one of the processing spaces and adapted so that the second mechanical element (67, 68) repetitively moves back-and-forth in synchronization with the back-and-forth movement of the first mechanical element (67, 68) in the same direction as the direction in which the first mechanical element (67, 68) moves back-and-forth; and the first and second mechanical elements being adapted to cooperate with each other to subject the fibrous web lying in each of the processing spaces during one of the forward and backward movements to ultrasonic processing, and at least one of the first and second mechanical elements is adapted to be spaced from the fibrous web in order to stop the ultrasonic processing during the other of the forward and backward movements. the outer peripheral surface being formed with at least one ultrasonic processing space being in communication with the inside and the outside of the drum; the first mechanical elements (67, 68) each being located inside the drum in one of the processing spaces so that the first mechanical element is adapted to repetitively move back-and-forth in a direction crossing the machine direction to pass transversely across the fibrous web; the second mechanical elements (67, 68) each being located outside the drum in one of the processing spaces and adapted so that the second mechanical element (67, 68) repetitively moves back-and-forth in synchronization with the back-and-forth movement of the first mechanical element (67, 68) in the same direction as the direction in which the first mechanical element (67, 68) moves back-and-forth; and the first and second mechanical elements being adapted to cooperate with each other to subject the fibrous web lying in each of the processing spaces during one of the forward and backward movements to ultrasonic processing, and at least one of the first and second mechanical elements is adapted to be spaced from the fibrous web in order to stop the ultrasonic processing during the other of the forward and backward movements.