To perform appropriate two folding operation while effecting gravure printing by forming an envelope main body through the welding of peripheries of front and rear two sheet films, and performing printing on one film, and forming both films of a composite film being coated or laminated with synthetic resin.

An envelope main body comprising of a synthetic film is formed into a bag by being thermally welded both the side edges and bottom edge of both strips 1a, 1b. Also, on one strip 1a, a destination of delivery or and address is indicated by giving mat ink through gravure printing, and a postal code column is formed thereon. Furthermore, the other strip 1b is maintained as being transparent by giving no gravure printing. Both strips 1a, 1b are formed of a composite film. Namely, it is formed by a coating resin consisting of nonstretched polypropylene on the inside surface of a main body film consisting of biaxially stretched polypropylene. On the other hand, a sealing strip 4 is formed by allowing one strip 1a to extend upwardly of an opening 5 from the other strip 1b, and then a release agent 7 is applied on one surface side by an adhesive 6.