A method for sealing-in a gas in a gas filling compartment 5 of a bag having a
cut-in 6 formed near the upper edge of the gas filling compartment. In the method, two edges of the bag with the contents inserted therein are gripped by
grippers and suspended, the blow-in port of a gas (air) blow-in
nozzle is brought to contact the
cut-in, the back surface side of the bag is supported by a backing member, gas (air) is blown into the gas filling compartment, and the position below the
cut-in is held from both sides of the bag by a gas cut-off
grippers while the gas blow-in continues, so that the gas inside the gas filling compartment is not allowed to escape from the cut-in, and then the entire bag mouth, including the cut-in, is sealed.