An image forming apparatus includes an image forming mechanism configured to contain a developer and to form an image with the developer and a developer supplier configured to supply the developer to the image forming mechanism. The developer supplier includes a developer container, a conveyer, a sensor, an agitator, and a driver. The developer container contains the developer. The conveyer conveys the developer from the developer container to the image forming mechanism. The sensor is disposed at a predefined position on an interior wall of the developer container and detects the developer at the predefined position in the developer container. The agitator rotates to agitate the developer and includes a paddle formed in a plate-like shape and including an opening through which a part of the developer slips and avoids being paddled by the paddle. The driver drives the conveyer and the agitator at a variable speed.