To eliminate the need of enclosing a return envelope in another return envelope to save a postal charge required for the cost and weight of the enclosed envelope by providing an envelope serving as both a sending envelope and a return envelope.

An envelope has one face on which a part for attaching a stamp and a part for displaying an address are formed. In the envelope, a space for stamp attachment and postmarking is cut out from the one face 10A to form a stamp attaching column 11, and an address display part 12 that is made transparent or available for write-in is also formed, and a dividing line 131 dividing a seal strip 13 into a sending seal part 133 and a return seal part 134 is displayed or formed, so that the returnable envelope is provided.