A developer cartridge includes a casing; and a sub-unit. The casing has a first opening and a developer chamber. The sub-unit is detachably attached to the developer chamber through the first opening. The sub-unit includes a cap; a developer container; and an agitation member. The cap seals the first opening. The developer container is disposed in the developer chamber and hermetically accommodating developer therein. The agitation member is disposed in the developer chamber. The agitation member has a first end portion and a second end portion. The agitation member is rotatable relative to the casing about an axis. The axis extends between the first end portion and the second end portion in an axial direction.