To provide a mailbox with a delivery prevention plate that can be attached to the mail box easily but cannot be detached easily unless the door of the mailbox is opened.

The mailbox delivery prevention plate mounting groove 16 is formed in the front side 14A of the door 14 attached to the mailbox body 2 of the mailbox 11 so as to be rotatable. A first locking projection 18 is formed at one end 16A of the mailbox delivery prevention plate mounting groove 16 whereas a second locking projection 20 is formed at the other end 16B thereof. A flap 24 is attached to an upper part of the door 14 of the mailbox body 12 so as to be rotatable. A rib 30 is formed on the plate body 28 of the mailbox delivery prevention plate 26 to be attached to the mailbox 11. A first locking body 32 projects from one end of the rib 30 whereas a second locking body 34 projects from the other end of the rib 30.