Abstract: A developing device effects desirable development when the amount of charging of toner, q (C/g), the amount of toner deposited on the surface of a developing roller per unit area, m (g/cm.sup.2), the speed of movement of the surface of the developing roller, v (cm/sec), the available length of the developing roller, l (cm), and the magnitude of electric resistance between the surface of the developing roller and a power source for developing bias, R (.OMEGA.), are adjusted so as to satisfy the formula, .vertline.q.vertline.m.ltoreq.100/vlR. The developing device produces a developed image desirable in quality and free from background fogging when the magnitude of a developing electric current, I (A), is adjusted so as to satisfy the formula, R.ltoreq.200/I.