A method for sealing-in a gas in a bag that has therein a gas filling compartment, including the steps of: placing a blow-out port of a nozzle, which is connected to a pressurized gas supply source, against a cut-in or a hole of the bag and holding the back surface side of the bag with a backing member; blowing a gas from the nozzle into the gas filling compartment of the bag through the cut-in or the hole; gripping and thus closing the gas flow path in sealed portions surrounding the periphery of the cut-in or the hole by a blocking gripper while the gas blow-in continues, thus cutting off the flow of the gas between the cut-in or the hole and the inside of the gas filling compartment; and then sealing the cut-in or the hole, thus allowing the gas to be sealed in the gas filling compartment.