To provide a security envelope showing no excessive cost even though its application is for security and in addition showing a high convenience in actual users (a sender and a receiver).

The security envelope includes upper and lower envelope pieces 11, 12 having at least two sides stitched to make a cylindrical envelope, a sender copy piece 13 arranged at the upper side of the upper envelope piece acting as a copy of the sender, a receiver copy piece 14 arranged on the upper side of the upper envelope piece acting as a copy of the receiver, a carbon 15 for selectively copying information described in the sender copy piece to the receiver copy piece and/or the upper envelope piece, and a flap portion 16-2 arranged at least on one end side of the upper or lower envelope piece having one side formed longer than the other side, and further includes a seal structure 17 for sealing the flap, and a seal opening cutting line 18 formed on the upper envelope piece placed at the position including almost of the receiver copy piece or a shape including almost of the receiver copy piece.