To enable an envelope for returning to be simply and easily prepared and mailed by improving a conventional reciprocating type envelope.

This sending/returning mail envelope to be used includes an envelope for sending and the envelope for returning. In the envelope, an unsealing section, on the front side of which an address and a name for the going out destination are listed, and on the rear side of which an address and a name for the returning destination are listed, is bent and pasted. At the time of returning, the unsealing section is unsealed, and the content of the envelope is taken out, and the unsealing section is pasted to the rear side so that the returning destination of the unsealing section may be exposed, and is returned. The sending/returning mail envelope to be used is constituted in this manner.