An image forming apparatus forming an image on a recording material includes an image bearing member; a rotary rotatably provided in a main assembly of the apparatus; a locking member; and a developing cartridge detachably mounted to the rotary, wherein the developing cartridge includes a developer carrying member for developing a latent image formed on the image bearing member, a demountably mounted covering member for covering a surface of the developer carrying member, and an engaging portion which is movable between a first position in which it is projected beyond an outer surface of the covering member to engage with the main assembly locking member when the rotary is rotated in a forward direction which is a direction of rotation during image formation, and a second position in which it is retracted from the outer surface of the covering member.