An image forming apparatus is described. The image forming apparatus includes a first casing accommodating a developing agent and having a first opening, an agitating member rotatably provided in the first casing and agitating the developing agent, a detection device outputting a signal in response to receiving of detection light passing through the first casing, and a control unit detecting an amount of the developing agent accommodated in the first casing and the position of the agitating member on the basis of the signal output from the detection device.