To provide a square-shaped envelope capable of simply and beautifully storing not only a letter with a small thickness but also a letter with a heavy thickness without much expense in time and effort.

The square-shaped envelope has an opening part on its one piece, and the opening part can be sealable with a lid piece. Gores are formed on side parts facing each other on the sides of the opening part, and gaps for forming a thickness are formed on corner parts between a bottom part facing to the opening part and the side parts where the gores are formed. It is possible thereby to simply and beautifully store not only the letter with a small thickness but also the thick one without much expense in time and effort.