1. An image forming apparatus comprising a developing means of putting developing material to sticking to an image holder, so as to form an image corresponding to an image data, a developing material supplying means of supplying said developing material to said developing means, a first electricity source for impressing said developing means with a first voltage, a second electricity source for impressing said developing material supplying means with a second voltage, an image density detecting means of detecting image density from said image data, and a controller for controlling each supply of said first electricity source and said second electricity source according to said image density detected by said image density detecting means.