A bag made of an outer bag (3) and an inner bag (2). The inner bag, inside the outer bag, has an opening in the upper edge, and the inner and outer bags are sealed along the upper edges, on either side of the respective bags, to form sealed portions (8) and (9), and the inner bag and outer bag are further sealed together at two lateral side edges and lower edge to form sealed portions (11) and (12) and sealed portion (13), thus forming gas filling compartments (14) and (15). Cut-ins (18) and (19) are formed in the upper corners of the outer bag, and supplemental sealed portions (16) and (17) connected to the sealed portions (8) and (9) are formed. Air (gas) is blown into the gas filling compartments (14) and (15) through the cut-ins and then the overall bag mouth including the cut-ins is sealed.