To accurately detect developer residual amount regardless of an image forming mode with respect to an electrophotographic image forming device having plural image forming modes and switching the frequency of developing bias voltage at the time of writing an image in accordance with the image forming mode.

An antenna 54 is arranged in parallel with the developing sleeve 52 of a developing device 5, and a detection circuit 56 is connected to the antenna 54. At the time of detecting developer residual amount, specified developing bias voltage is applied to the sleeve 52 from a developing bias power source 55 and toner residual amount between the sleeve 52 and the antenna 54 is detected as the change of electrostatic capacity by the circuit 56. In such a case, the developing bias voltage having single frequency is applied to the sleeve 52 regardless of the image forming mode.