In an image forming apparatus, a revolver type developing device has a plurality of developing units and revolves to sequentially bring them to a developing position facing an image carrier on which a latent image is electrostatically formed. The revolver promotes easy replacement and safe handling of the developing units and can be accurately positioned relative to the body of the image forming apparatus. Despite that the revolver is miniature, it prevents developers from leaking or flying off from the developing units and contaminating the inside of the apparatus or being mixed with each other.