A developing device having a developing gap detecting function includes a photosensitive medium, a developer conveyer, a DC power source, an AC power source, a current detecting unit and a controller. The current detecting unit detects a DC current flowing on the developer conveyer when toner from the developer conveyer is attached to a desired area of an electrostatic latent image on a photosensitive body by enhancing a voltage outputted from a power supply gradually at proper intervals. The controller obtains a developing gap between the photosensitive medium and the developer conveyer based on current values detected by the current detecting unit. In accordance with the developing device having the developing gap detecting function, the developing gap is sensed and developing voltage is easily and correctly calculated to control image forming conditions. As a result, an image quality improves.