A highly reliable developing device, process cartridge, device unit, and image forming apparatus in which a developer is not spattered to the outside of the developing device even when the developing device where a developer is preset is included with shipment in an image forming apparatus main body. This developing device comprises a developer container for containing a developer, and a toner replenishing portion which is connected with a toner conveyance portion installed in the image forming apparatus main body and which is communicated with the developer container via an opening portion. Such a developing device is provided with a sealing member for sealing the opening portion until use of the developing device on the image forming apparatus main body is started.