In the manufacture of packing containers by external processing, sealing and cutting of tubular packing material the tube contents are made use of as a hold-on in the shaping work. To prevent the eclosed quantity of air from being pressed out in this process through the upper open end of the tube a sealing unit (14) present in the tube is used which comprises a movable sealing holder (18) which in accordance with the invention is pressed by means of the pressure of the enclosed quantity of air in a balanced manner so as to form a seal against a sealing surface (22). On the periphery of the sealing holder a sealing (24) of flexible material is provided which is resting aginst the inside of the material tube.