To provide an envelope which can be deformed into a stand and expanded freely.

An envelope 1 can be expanded freely and deformed into a stand. In an expanded state of the envelope, a top-and-bottom face 10 having a first reference line 11 which traverses a center, a tongue part 20 extending toward an upper part of the top-and-bottom face 10, an engaging piece 21 projecting from an upper end of the tongue part 20, a folding part 30 extending toward a lower part of the top-and-bottom face and having a second reference line 31 which traverses a center, and right-and-left guide pieces 40, 50 extending from the first reference line and the right-and-left sides of the top-and-bottom face respectively are provided. From the expanded state, the envelope 1 can be easily deformed into an envelope or a stand.