A unit for sealing filter bags for products for infusion, the unit comprising at least: - a shaped revolving roller with circumferential and radial tracks (3, 4) that act as guides to make longitudinal and transversal contact seals on a web of filter paper folded onto itself and moving under the roller, using - sealing sonotrode means arranged in sealing position under the roller and close to the moving web in such a way as to form a tubular web of filter paper with a succession of substantially flattened filter bags formed on it; the sealing sonotrode means comprising a first and a second sonotrode unit (6a, 6b) mounted side by side in such a way as to perform two separate sealing operations, resulting in a longitudinal seal and a transversal seal, acting in conjonction with respective plates (6c,6d) that come into contact with the passing product and with the tracks (3, 4), each sonotrode unit (6a, 6b) being supported by a plate that is linked to a shaft protruding from a fixed frame; the unit being characterised in that the sealing sonotrode means are operated upon by fluid-driven means designed to control, through appropriate means, the variable constituted by the contact force and/or the distance between the sealing means and the roller; the fluid-driven means comprising two cylinders (7c, 7d) linked at one end to the fixed frame and linked at the top to the plate in such a way that a first cylinder permits adjustment of the contact force of the first sonotrode that makes the longitudinal seal on the web, and the second cylinder permits adjustment of the gap between the working end of a second sonotrode and the respective track facing the second sonotrode. - a shaped revolving roller with circumferential and radial tracks (3, 4) that act as guides to make longitudinal and transversal contact seals on a web of filter paper folded onto itself and moving under the roller, using - sealing sonotrode means arranged in sealing position under the roller and close to the moving web in such a way as to form a tubular web of filter paper with a succession of substantially flattened filter bags formed on it; the sealing sonotrode means comprising a first and a second sonotrode unit (6a, 6b) mounted side by side in such a way as to perform two separate sealing operations, resulting in a longitudinal seal and a transversal seal, acting in conjonction with respective plates (6c,6d) that come into contact with the passing product and with the tracks (3, 4), each sonotrode unit (6a, 6b) being supported by a plate that is linked to a shaft protruding from a fixed frame; the unit being characterised in that the sealing sonotrode means are operated upon by fluid-driven means designed to control, through appropriate means, the variable constituted by the contact force and/or the distance between the sealing means and the roller; the fluid-driven means comprising two cylinders (7c, 7d) linked at one end to the fixed frame and linked at the top to the plate in such a way that a first cylinder permits adjustment of the contact force of the first sonotrode that makes the longitudinal seal on the web, and the second cylinder permits adjustment of the gap between the working end of a second sonotrode and the respective track facing the second sonotrode.