An image forming apparatus includes a main unit; and a developer cartridge detachably attached to the main unit. The developer cartridge includes a rotational body having a rotational axis and rotatable in a rotation direction; and an extended portion formed on the rotational body to extend along the rotational axis, the extended portion rotatable around the rotational axis. The main unit includes a driving unit configured to rotate the rotational body in the rotation direction; a detecting unit configured to detect a movement of the extended portion and an extending amount of the extended portion; and a determining unit configured to: determine whether the developer cartridge is a new product according to a presence of a movement of the extended portion detected by the detecting unit; and determine type of the developer cartridge based on the extending amount of the extended portion detected by the detecting unit.