A transfer belt unit mountable to and demountable from an apparatus main body of an image forming apparatus, the transfer belt unit including: a transfer belt being in contact with a photosensitive drum of a drum cartridge of the image forming apparatus in a state where the transfer belt unit is mounted to the apparatus main body, the transfer belt being configured to transfer a toner image on the photosensitive drum to a sheet conveyed between the photosensitive drum and the transfer belt and to convey the sheet conveyed between the photosensitive drum and the transfer belt toward a fixing unit of the image forming apparatus in the state where the transfer belt unit is mounted to the apparatus main body, and a belt memory in which information about the transfer belt unit is stored.