In an image forming apparatus having a drum unit and a developing unit attachable to or detachable from an apparatus body, when the drum unit is inserted into or removed from the apparatus body, operation in the following first to third sections occurs in order or reverse. The first section is a predetermined section on a removal side from the innermost position of the drum unit. In the first section, the developing unit is inserted/removed integrally with the drum unit. In the second section, only the drum unit is inserted or removed. Meanwhile, the developing unit gradually rotates to switch the contact or separated states in conjunction with the insertion or removal of the drum unit. In the third section, only the drum unit is inserted into or removed from the apparatus body while the developing unit is separated.