Disclosed is a sealing apparatus wherein, as a drum (5) and a rotary base (6) are rotated together, a follower (10a) of a rocking support member (9) rockably provided on the outer periphery of the rotary base (6) is guided along a cam groove (15c) of a cam member (15) to drive the rocking support member (9) to rock. When the rotary drum (5) reaches a predetermined rotational position, an anvil (14) supported by the rocking support member (9) is urged onto a horn (8), and then, when it reaches another rotational position, the anvil (14) is moved away from the horn (8). Since the anvil (14) is driven by the rocking motion of the rocking support member (9), entire operation becomes quite simple, and a continuous soft work can be certainly clamped between the horn (8) and the anvil (14).