An image forming apparatus is capable of preventing with certainty the loading of a developing device an incorrect loading portion from being operated. The image forming apparatus includes a latent image holding body forming an electrostatic latent image, a developing device developing the electrostatic latent image to form a visible toner image, the developing device being detachably loaded in the image forming apparatus, a transfer device transferring the developed image onto a printing medium and a fixing device for fixing the transferred image. The image forming device can prevent an incorrect developing device loading in the image forming apparatus from being placed in a development enabled condition and can detect an active state of prevention.