An image forming apparatus of the present invention includes a photoconductive drum and a developing device for developing a latent image formed on the drum with a developer. The developing device includes a developing roller facing the drum via an opening formed in the casing of the developing device. A feeding device feeds a controlled gas to a position upstream, in the direction of rotation of drum, of a developing position where the developing device operates. A first switching device selectively causes a developer layer deposited on the developing roller to contact the developing zone of the drum in an image formation condition or to part from the developing zone in a stand-by condition. A sealing device maintains, in the image forming condition, a gap between the drum and the casing at a position downstream of the developing zone in the direction of rotation of the drum or seals the gap in the stand-by condition.