To use a bag as cushion material for transportation of articles, heat insulation sheet for leisure or disaster relief, or transportation vessel for beverage and the like with fluid such as air and water filled therein, and allow charge/discharge from/into the whole bag through one charge/discharge port by being communicated through slits of partition walls in a manner restraining expansion of each bag wall and retaining its shape in flat as being filled with fluid.

The airtight bag is formed by welding bag walls 21, 22 consisting of material having a heat fusible property in its internal surface by peripheral lines 31, 32, 33, 34, and provided with a charge/discharge port 8, in which a plurality of separation strips, or tube-like matter and sheet-like matter each having a heat-melt property are interposed between the two opposite walls 21, 22, and these interposed matters are respectively melted to each inner surface of the bag walls 21, 22 by a plurality of joint lines 41, 42 opposed to each other in parallel with a opposite pair of the peripheral lines 31, 33 of the bag 1, and further, these interposed matters restrain both bag walls 21, 22 for the purpose of forming partition walls 6 for separating the interior of the bag 1 before forming a plurality of bag cambers, and still further, a plurality of slits 7 are made at respective partition walls 6 in the direction intersecting with each joint line 41, 42, thus achieving a communication between each bag chamber and the charge/discharge port 8 provided at a part of the bag 1.