PURPOSE: To make a bag gas- and moisture-permeable completely all the time, yet permitting its water-barrier feature to be secured by forming tiny through openings in the film consisting mainly of polyolefin resins and the thermoplastic resins slightly miscible therewith.

CONSTITUTION: Two sheets of gas-permeable film 2 and 3 are obtained by a method wherein the resin materials consisting mainly of polyolefin resins and the thermoplastic resins slightly miscible therewith are melted and kneaded, the resulting mass is extrusion molded into film and this film is treated with heat over a predetermined temperature range at non-shear rate, followed by quenching and then drawing. The gas-permeable films 2 and 3 have a number of tiny openings formed therethrough, indicating a high degree of gas- and moisture-permeability. By reason of these tiny openings, however, the gas- permeable films 2 and 3 keep out liquid such as water with a high degree of water-barrier feature. Vapor escapes to the outside through the tiny openings of these films and will not stay in a moisture-permeable, water-barrier packaging bag. Conversely, if water is splashed on such bag from the outside, it is kept from entry into the bag.