A method for charging a bag equipped with a gas compartment portion with gas, wherein said method is for a bag formed integrally therein with a gas compartment portion extending in a longitudinal direction in a sealed side edge portion of the bag and having a gas inlet that is formed in a film sheet constituting the gas compartment portion and places an exterior of the bag in communication with an interior of the gas compartment portion, and said bag is conveyed along a predetermined bag conveying path, wherein said method, while the bag is on the bag conveying path, carries out a gas filling step and subsequently a gas charging step, in which: in the gas filling step, a discharge outlet of a gas injection nozzle (23,24) connected to a pressurized gas supply source is aligned with the gas inlet, gas is injected from the gas injection nozzle (23, 24) into the gas compartment portion through the gas inlet, and, while the injection is in progress, a blockage region configured in the vicinity of the gas inlet is gripped by a gas blocking gripper on both sides of the bag to block fluid communication of gas between the gas inlet and the interior of the gas compartment portion; and in the gas charging step, the bag, with the blockage region being clamped by the gas blocking gripper, is pressure-held on both sides at a location of the gas inlet or at a location in the vicinity of the gas inlet with hot plates to thereby seal the gas inlet or an area in the vicinity of the gas inlet and encapsulate the gas in the gas compartment portion, and characterised in that prior to the gas filling step, said method carries out a preliminary gas injection step that, while the bag is being on the bag conveying path, aligns a discharge outlet of another gas injection nozzle connected to a pressurized gas supply source with the gas inlet and injects gas from the other gas injection nozzle into the gas compartment portion through the gas inlet. in the gas filling step, a discharge outlet of a gas injection nozzle (23,24) connected to a pressurized gas supply source is aligned with the gas inlet, gas is injected from the gas injection nozzle (23, 24) into the gas compartment portion through the gas inlet, and, while the injection is in progress, a blockage region configured in the vicinity of the gas inlet is gripped by a gas blocking gripper on both sides of the bag to block fluid communication of gas between the gas inlet and the interior of the gas compartment portion; and in the gas charging step, the bag, with the blockage region being clamped by the gas blocking gripper, is pressure-held on both sides at a location of the gas inlet or at a location in the vicinity of the gas inlet with hot plates to thereby seal the gas inlet or an area in the vicinity of the gas inlet and encapsulate the gas in the gas compartment portion, and characterised in that prior to the gas filling step, said method carries out a preliminary gas injection step that, while the bag is being on the bag conveying path, aligns a discharge outlet of another gas injection nozzle connected to a pressurized gas supply source with the gas inlet and injects gas from the other gas injection nozzle into the gas compartment portion through the gas inlet.