To provide a round-trip envelope that performs both sending a letter and sending a reply by one envelope, and has a constitution in which when it is sent back to a right address for replying sealed for sending a reply.

The round-trip envelope has a letter-sending displaying area 4 for displaying information on the address for sending the letter on the front face side of an envelope body 21, and a reply-sending displaying area 5 for displaying information on the address for sending the reply on the back face side of the envelope 1 of a reply flap 23. When sending the letter, under a condition that the information on the address for sending the letter is visible, sealing is performed under a condition that a letter-sending flap 22 covers the reply flap 23 using the upper side of the envelope body 21 as a folding line 6. When sending a reply, under a condition that the information on the address for reply is visible, sealing is performed under a condition that a reply flap 23 covers the information on the address for sending the letter using the upper side of the envelope body 21 as the folding line 6.

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