A longitudinal continuous welding device for packaging machines, which comprises at least: - one supporting piece to support layers of material to be welded continuously; - two ultrasonic welders (2, 2') having respective sonotrodes facing opposite sides of said supporting piece horizontally; - two longitudinal paths between said sonotrodes and the supporting piece, along which the two groups of layers of material to be welded continuously, and simultaneously, by the respective ultrasonic welders (2, 2') advance inside the packaging machine, the sonotrodes and the supporting piece being unable to move in a direction of longitudinal movement of the layers of material; and - two positioning elements to arrange the welding device in a nonoperating position, in which the sonotrodes remain distanced horizontally from the supporting piece, or in an operating position for welding in which the sonotrodes are maintained very close to the supporting piece; characterised in that: - two elastic pushers consisting of spiral springs are provided which act on said ultrasonic welders (2, 2') to push the ultrasonic welders (2, 2') and the respective sonotrodes against the layers of material to be welded continuously which are supported on the opposite sides of the supporting piece; and - said spiral springs cause the ultrasonic welders (2, 2') and the supporting piece to exercise pressure on opposite sides of the two groups of layers of material to be welded continuously and enable separations between the ultrasonic welders (2, 2') and the supporting piece to vary at a very high frequency due to the action of the sonotrodes, whereby enabling the layers of material to be welded to advance continuously through said separations between the supporting piece and the sonotrodes as the sonotrodes conduct the welding of both groups of layers of material. - one supporting piece to support layers of material to be welded continuously; - two ultrasonic welders (2, 2') having respective sonotrodes facing opposite sides of said supporting piece horizontally; - two longitudinal paths between said sonotrodes and the supporting piece, along which the two groups of layers of material to be welded continuously, and simultaneously, by the respective ultrasonic welders (2, 2') advance inside the packaging machine, the sonotrodes and the supporting piece being unable to move in a direction of longitudinal movement of the layers of material; and - two positioning elements to arrange the welding device in a nonoperating position, in which the sonotrodes remain distanced horizontally from the supporting piece, or in an operating position for welding in which the sonotrodes are maintained very close to the supporting piece; - two elastic pushers consisting of spiral springs are provided which act on said ultrasonic welders (2, 2') to push the ultrasonic welders (2, 2') and the respective sonotrodes against the layers of material to be welded continuously which are supported on the opposite sides of the supporting piece; and - said spiral springs cause the ultrasonic welders (2, 2') and the supporting piece to exercise pressure on opposite sides of the two groups of layers of material to be welded continuously and enable separations between the ultrasonic welders (2, 2') and the supporting piece to vary at a very high frequency due to the action of the sonotrodes, whereby enabling the layers of material to be welded to advance continuously through said separations between the supporting piece and the sonotrodes as the sonotrodes conduct the welding of both groups of layers of material.