An ultrasonic sealer comprising a hollow shaft (1) having a longitudinal axis (2) and being rotatably arranged in a housing (3) for driven rotation about its longitudinal axis (2). The rotary sealer further comprises a sealing horn (8) arranged coaxially in the hollow shaft (1) and connected to a converter (6) and preferably also a booster (7). The horn (8) is provided with a peripheral sealing area (13) outside an axial end (13) of the hollow shaft (1). The horn (8) is connected to the hollow shaft (1) by means of at least one annular, metallic support (11) extending between an outer face (14) of the horn (8) and an inner face (10) of the hollow shaft (1). The at least one annular support (11) is corrugated in cross section such that at least two grooves and one ridge arranged there between are provided between the inner face (10) of the hollow shaft (1) and the outer face (14) of the horn (8).