There is related to an image forming apparatus for forming a two-color toner image on a photosensitive body and transferring a batch of the image to a transfer material. The apparatus has a rotating and moving electrophotographic photosensitive body. Successively along a rotating direction of the photosensitive body arranged are a first developing device using a two-component developing agent constituted of a magnetic carrier and a nonmagnetic toner and a second developing device using a one-component magnetic developing agent different in color from the nonmagnetic toner. The apparatus also has a control circuit for stopping an image forming operation when a trouble occurs in an apparatus body. To prevent the carrier of the first developing device from mixing in the second developing device, a drive mechanism is provided for detaching a magnet of the second developing device from the photosensitive body to a position at which the carrier adhering from the first developing device to the photosensitive body does not mix in the second developing device by means of a magnetic force of the second developing device when the image forming operation is stopped.