An image forming apparatus comprising: a latent image carrier configured to carry a latent image thereon; a developing unit disposed facing the latent image carrier to develop the latent image with developer; a developer container configured to contain the developer and to be attached to the image forming apparatus; a developer detector (190; 191; 193,197) configured to detect presence of the developer in the developing unit; a developer supply controller configured to prohibit supply of the developer from the developer container to the developing unit when the developer detector (190; 191; 193,197) detects that the developer is present in the developing unit; and a transferor configured to transfer the image formed on the latent image carrier onto an intermediate transfer member; characterised in that the image forming apparatus further comprises: a disengagement unit (140; 140A; 140B; 140C) configured to engage the intermediate transfer member with the latent image carrier and disengage the intermediate transfer member therefrom; and disengagement detector (195; 195A; 196; 199) configured to detect whether or not the intermediate transfer member is disengaged from the latent image carrier, wherein the developer supply controller is configured to supply the developer from the developer container to the developing unit after the disengagement detector (195; 195A; 196; 199) detects that the intermediate transfer member is disengaged from the latent image carrier. a latent image carrier configured to carry a latent image thereon; a developing unit disposed facing the latent image carrier to develop the latent image with developer; a developer container configured to contain the developer and to be attached to the image forming apparatus; a developer detector (190; 191; 193,197) configured to detect presence of the developer in the developing unit; a developer supply controller configured to prohibit supply of the developer from the developer container to the developing unit when the developer detector (190; 191; 193,197) detects that the developer is present in the developing unit; and a transferor configured to transfer the image formed on the latent image carrier onto an intermediate transfer member; characterised in that the image forming apparatus further comprises: a disengagement unit (140; 140A; 140B; 140C) configured to engage the intermediate transfer member with the latent image carrier and disengage the intermediate transfer member therefrom; and disengagement detector (195; 195A; 196; 199) configured to detect whether or not the intermediate transfer member is disengaged from the latent image carrier, wherein the developer supply controller is configured to supply the developer from the developer container to the developing unit after the disengagement detector (195; 195A; 196; 199) detects that the intermediate transfer member is disengaged from the latent image carrier. a disengagement unit (140; 140A; 140B; 140C) configured to engage the intermediate transfer member with the latent image carrier and disengage the intermediate transfer member therefrom; and disengagement detector (195; 195A; 196; 199) configured to detect whether or not the intermediate transfer member is disengaged from the latent image carrier,