To inexpensively send a card-type CD in such a manner as to prevent the card-type CD from being cracked or broken when an envelope, in which the card-type CD is enclosed, passes through a mail sorting machine.

In the one side opening type envelope, an envelope feeding direction of an envelope sorting device is taken as a long side direction of the envelope, and one of a pair of opposed long sides of the envelope is opened. The envelope has a card inserting part 10 where the card-type CD 11 is disposed in such a manner that a long side 12 of a card is orthogonal to the long side direction of the envelope, and the inserting part 10 is provided near a center between short sides 14 of the envelope.