An apparatus to detect the remaining amount of developer in a developing unit is provided. The apparatus includes a light emitting prism and a light receiving prism placed opposite to each other in a developer store space of the developing unit and forming a light emitting path and a light receiving path, respectively. A light emitting element irradiates light through the light emitting prism, and a light receiving element receives a light signal through the light receiving prism. An antistatic agent is coated on a light emitting window of the light emitting prism and a light receiving window of the light receiving prism, respectively, to substantially prevent developer from attaching on the light emitting window and the light receiving window. Because the developer is not stuck on the light receiving window and the light emitting window to which the light signal is transmitted, an accurate light signal may be transmitted from the light emitting window to the light receiving window, thereby substantially preventing printing defects caused by detection errors from occurring.