A cartridge detachably attachable to an image forming apparatus which includes a main body, a driving unit and a detecting unit, includes: a housing that is configured to accommodate a developer therein, and includes a first side wall and a second side wall opposed to the first side wall in a longitudinal direction; a passive unit that is configured to receive a driving force from the driving unit, is mounted on the first side wall, and is rotatable around a first axis line parallel to the longitudinal direction; and a detected body mounted on the first side wall and including a detected part which is detected by the detecting unit. The detected body advances outwards in the longitudinal direction with respect to the first side wall and retracts inwards in the longitudinal direction with respect to the first side wall by the driving force received by the passive unit.