To provide an envelope which allows a user to know the history of sealing and unsealing and does not limit an address at reuse, a seal, and its usage.

An envelope is formed of an envelope body equipped with a window for reading address information and sender information, and a flap seal formed by lamination of two or more sheets. The flap seal is provided with a plurality of perforations. In sealing, one piece of a pasteboard for release of a flap seal underlayer sheet is cut and teared off into a strip shape partially and in stages along the perforations, and then an adhesive surface of a middle layer sheet shaped roughly the same as the strip shape appears and it sticks to the rear of the envelope and they are sealed. In opening, one piece of a flap seal upper layer sheet shaped roughly the same as the strip of the middle layer sheet stuck to the rear of the envelope is cut and teared off partially and in stages, whereby they are opened. In the trace of opening, the middle layer sheet stuck to the envelope leaves a strip-shaped mark and a nonadhesive surface remains on the surface side. There are provided such a configuration of an envelope and a flap seal, and usage thereof.