PURPOSE: To provide a developing unit which makes a toner layer thin, applies uniform electrification and, thus, obtains a highly-reliable good quality image.

CONSTITUTION: The unit is provided with a developing roller 2 for developing an electrostatic latent image on a photoreceptor drum 1, a supply roller 3 for supplying the specific toner T to the developing roller 2, a toner replenishment part 4 for replenishing the supply roller 3 with the specific amount of the toner T, and a toner blade 5 for restricting the amount of the toner T sticking to the developing roller 3. Negative potential which is smaller in absolute value than that applied to the developing roller 2 is constantly applied to either the supply roller 3 or toner blade 5.