An elongated web of bag-making material (S) is bent into a tubular form by a former (2), transported along a path (A), is filled with articles (W) and sealed by a transverse sealer (10) transversely to its direction of motion. A pair of seal jaws (20) supported by rotary arms (12) are rotated in mutually opposite directions by a power-communicating mechanism (P) which includes a trajectory-forming mechanism (H) such that the seal jaws (20) will each travel on a generally D-shaped trajectory with a linear section along the path (A) of the bag-making material (S). Cramming devices (50) are adjustably attached to the rotary arms (12) and stroke the tubularly shaped bag-making material (S) from both sides in forward-backward direction (X) without substantially closing the bag being formed such that the articles (W) can be more efficiently crammed with empty spaces among them effectively reduced. Stripping rods (42) are attached to the seal jaws (20) for stripping down the articles being dropped into the bag immediately before the transverse sealing and after the cramming by the cramming devices (50) so as to prevent them from becoming caught between the seal jaws (20).