The invention relates to a device for gas filling of a duct (7) in a container (1) of a collapsible type, said duct (7) being defined by a first and a second side wall (2) of the container (1), the side walls (2) being joined along a common connecting portion (4), and said duct (7) comprising an inlet (15) arranged in the first side wall (2). The device comprises an abutment (21) and a gas module (26) with an outlet (28), which is arranged in a surface of the gas module (26) facing the abutment (21), and a clamping means (29) arranged outside the outlet (28), which clamping means (29) is applicable to the abutment (21) for clamping the container (1), and which outlet (28), in connection with the clamping of the container (1), is applicable to said inlet (15) for supply of gas to the duct (7) of the container. The device is characterized by a groove (30) formed in said surface of the gas module (26), which groove surrounds the outlet (28) and is positioned radially inside said clamping means (29), said groove (30) being arranged to prevent, when supplying gas to the duct (7) intended to be filled with gas, build-up of a pressure above atmospheric on the side, facing the gas module, of said first side wall (2) surrounding said inlet (15). The invention also relates to a method for filling such a duct with gas.