To prevent an envelope window from being displaced from an address of an inserted document.

A developed sheet having a rear sheet 2, a seal piece 3, a left adhering piece 4 and a right adhering piece 5, which are continuously arranged all around front sheet 1 with a window, the rear sheet 3 is overlapped at a line (c), the left adhering piece 4 is overlapped at a line (d), the right adhering piece 5 is overlapped at a line (e) on the inner surface of the front sheet 1 while being folded to each other, the overlapped portions of the folded members adhere to each other to form a bag and the envelope has a seal piece 3 folded at a line (f). Before forming the bag, a glue applying part 11 is arranged near the inner folding line where the front sheet 1 and the rear sheet 2 are continuously arranged. A scaled-down segment A×B is formed concurrently inside the envelope through utilization of a notch 8 against the glue applying part 13.