To make the productivity extremely high, and at the same time, make a mismatch of a sent information hard to generate by a method wherein a bonding part is provided between confronted surfaces of a section-form article, and a bonding is performed on the bonding part.

A bonding part 20 is respectively formed with a pressure- sensitive adhesive which firmly bonds to a degree wherein a peeling is impossible, on the peripheral edge parts of three edges without a folding line 5' on a paper surface 2a and a paper surface 3a which are confronted, on the peripheral edge parts of three edges without the folding line 5 on a paper surface and a paper surface 2b which are confronted, and on the peripheral edge parts of three edges without the folding line 5" of a paper surface 3b and a paper surface 4b which are confronted. At the same time, at a specified location on the inside of the bonding part 20 which is formed into a rectangular paper piece 8, a perforation 10 which is in parallel with respective edges of the rectangular paper piece 8, is formed. By folding the folding lines 5, 5' and 5" into a section-form in such a manner that a postage stamp surface may be exposed, a section-form article is made, and by pressing the section-form article, respective bonding parts 20 are bonded to each other, and this sealed letter is formed.