To provide a developing device capable of forming a toner thin layer stably and excellently on a developer carrier.

The developing device has a developer, the rotatable developer carrier, a conductive developer regulation member for regulating the developer by abutting it on the developer carrier, a voltage application means for generating a DC electric field between the developer carrier and the conductive developer regulation member, a controller for controlling the rotation of the voltage application means and the developer carrier, a current value measuring instrument for measuring a current value flowing through the developer regulation member and a computer for analyzing the value measured by the current value measuring instrument and sending a proper electric signal to the controller. The voltage application means applies the voltage to the conductive developer regulation member so as to vary gradually, the current value measuring instrument measures the current flowing through the conductive developer regulation member, the computer selects specific voltage from the measured values, and the controller operates the voltage application means so as to apply this selected voltage to the conductive developer regulation member.