An image forming apparatus, including a main body and a cartridge configured to be detachably attached to the main body, is provided. The cartridge includes a photosensitive member configured to carry an image formed in a developer agent thereon, a movable mechanism, and a developer roller configured to supply the developer agent to the photosensitive member. The movable mechanism is configured to shift between a first state, in which the developer roller is placed in a contact position to contact the photosensitive member, and a second state, in which the developer roller is placed in a separated position to be separated from the photosensitive member. The main body includes a detectable member configured to detect a state of the movable mechanism between the first state and the second state.