To prevent sterile air at a positive pressure from entering a bag to interfere with filling of contents when the bag is set on a clamp of a filling machine in aseptically filling an inner bag of a bag-in-box(BIB) or prevent a problem such as liquid splash from occurring when removing the contents in an ejection port by air flush.

When an inner bag 1 of a BIB is set on a clamp 15 of an aseptic filling machine, an internal face 2a of the bag is pressed against a lower part of an ejection port 3 by a pressing tool 11 to close the ejection port 3, a cap 5 is removed in this state (figure (a)), a filling nozzle 20 is fitted into the ejection port 3, and the pressing tool 11 is lowered to have contents 10 filled (figure (b)). After a regulated amount has been filled, the pressing tool 11 is raised to bring the internal face 2a of the bag close to the lower part of the ejection port 3, forming a gap between the ejection port and the internal face of the bag to be 3 to 5 mm, the contents attaching inside the ejection port are removed by air flush (figure (c)), the filling nozzle is removed from the ejection port, and the cap is put on, completing aseptic filling.