An image forming apparatus, comprising: a developing device having a developer bearing member that is disposed so as to be opposed to an image bearing member on which an electrostatic latent image is formed and, is configured to supply at a region thereof opposed to the image bearing member, a developer to the image bearing member; a support frame that is capable of disposing the developing device selectively at a mounting/demounting position where the developer bearing member is separated from the image bearing member and at a developing position where a developer can be supplied to the image bearing member; and a developer supply mechanism (5, 6) that is configured to supply a developer to the developing device, wherein around a connection portion of the developer supply mechanism (5, 6) at which the developer supply mechanism (5, 6) is connected to the developing device, a shutter member that is configured to open/close a replenishment port formed at the connection portion is fitted, and the shutter member is configured to open the replenishment port in tandem with movement of the developing device from the mounting/demounting position to the developing position and to close the replenishment port in tandem with movement of the developing device from the developing position to the mounting/demounting position, where a direction in which the developing device is moved is different from a direction in which the shutter member performs the opening/closing of the replenishment port, the shutter member is a cylindrical member into which the connection portion is rotatably inserted and that has an opening portion formed on an outer peripheral surface thereof at a position that is to coincide with a position of the replenishment port, characterized in that the developing device has a guide portion that has a substantially oval section and into which the shutter member is inserted, and the developing device is configured to be moved to and fro between the maunting/demounting position and the developing position, so that the shutter member is relatively and horizontally moved in a horizontal direction with respect to a main body of the image forming apparatus while rotating within the guide portion and thus opens/closes the replenishment port. a developing device having a developer bearing member that is disposed so as to be opposed to an image bearing member on which an electrostatic latent image is formed and, is configured to supply at a region thereof opposed to the image bearing member, a developer to the image bearing member; a support frame that is capable of disposing the developing device selectively at a mounting/demounting position where the developer bearing member is separated from the image bearing member and at a developing position where a developer can be supplied to the image bearing member; and a developer supply mechanism (5, 6) that is configured to supply a developer to the developing device, wherein around a connection portion of the developer supply mechanism (5, 6) at which the developer supply mechanism (5, 6) is connected to the developing device, a shutter member that is configured to open/close a replenishment port formed at the connection portion is fitted, and the shutter member is configured to open the replenishment port in tandem with movement of the developing device from the mounting/demounting position to the developing position and to close the replenishment port in tandem with movement of the developing device from the developing position to the mounting/demounting position, where a direction in which the developing device is moved is different from a direction in which the shutter member performs the opening/closing of the replenishment port,