PURPOSE: To obtain an electrophotographic copying machine which is capable of expanding the density variable width of a line image, and also adjusting the copied image density in a state that a density variation of the line image and a density variation of a solid image are kept almost constant.

CONSTITUTION: A controlling circuit 10 for executing adjustment of picture density controls the charged potential on a photosensitive drum and also controls the developing bias potential, and the applied potential of the developing bias is secided so as to satisfying the relation of VBVo-200V (VB: developing bias potential, Vo: photosensitive body charged potential) in accordance with the charged potential of the photosensitive drum fed back by said detector 4. The charged potential on the photosensitive drum 1, which is adjusted in this way and is provided by a charger 2 is detected by the detector 4, and the developing bias of a potential value decided in accordance with said expression in the controlling circuit 10 is applied to a developing sleeve 8 from an electric power supply 9. That is to say, if the charged potential is 450V, the developing bias potential is adjusted to 250V, and if the former potential is 350V and 250V, the latter potential is adjusted to 100V and 50V, respectively, and absolute value of the potential difference between the charged potential and the developing bias potential is held at 200V.