To adjust bulk of articles to be packaged, which are put into a packaging material, to a constant level prior to a lateral seal processing.

A plurality of air-sacks 82... regulated to a predetermined height by regulating the pressure of gas fed thereinto are pushed against both sides of a cylindrical film (f), and it is so designed that bulk of articles W to be packaged, which are put into the cylindrical film (f), is adjusted to a constant level. In this case, a controlling circuit works to control the air volume in each of the air-sacs 82... by utilizing seal-checkers 9 between which the molded articles are sandwiched from the upper and lower sides thereof under a fixed pressure, and is composed of an input means 11 for inputting a detection signal released out of a sensor S for detecting the thickness (d) of each article W stored inside the cylindrical film, an external input means 12 for setting the thickness (d) according to the kinds of articles W and the size of bags, an operation means 13 for computing an air volume in each of the air-sacs 82... based on a signal released out of the external input means 12, and a comparison means 14 for comparing a detection signal released out of the input means 11 with a signal released out of the operation means 13 and controlling an electropneumatic proportioning valve 84 based on a signal difference therebetween.