To provide an envelope capable of easily opening without using a special tool and again using the envelope used as a forward mail to a reply mail.

In the envelope 1 formed by a front face paper 11 and a back face paper 15 and forming a flap 12 at the upper end of the front face paper 11, a partial flap 13 capable of being opened with a cutting line 12a is formed at a side end of the flap 12, and an opening piece 17 capable of cutting with cutting lines 17a, 17b is formed on an end of the back face pater 15 at a partial flap 13 side.