A rotary developer for an image forming apparatus. Development cartridges accommodated in a rotary unit at a predetermined set position are selectively moved between development and non-development positions. A self-attachment locking mechanism including a pre-pressure applying mechanism self-locks a development cartridge at the predetermined set position when the development cartridge is inserted by a predetermined insertion amount that does not reach the predetermined set position. The pre-pressure applying mechanism includes a pressing member on one of the rotary unit and development cartridge and a pressed member on the other of the rotary unit and development cartridge. The pressed member includes gently sloped and steeply sloped surfaces. When the development cartridge is inserted by the predetermined insertion amount, the pressing portion transitions from contact with the gently sloped surface to contact with the steeply sloped surface.