A developing device for developing an electrostatic image formed on an image carrier, includes a liquid developing agent, wherein toner particles are dispersed in a liquid solvent, and wherein a viscosity of the liquid developing agent is in a range 100 mPas to 1000 mPas, inclusive, a developing container for containing the liquid developing agent, a developing agent carrier for carrying the liquid developing agent within the developing container to the image carrier, wherein the developing agent carrier is disposed so as to form a gap between the developing agent carrier and the image carrier; and a developing bias voltage power supply for applying a developing bias voltage, which superimposes a DC voltage component and an AC voltage component to the developing agent carrier, wherein a peak voltage of the AC voltage component is in a range of 0.5 kV to 2 kV, inclusive, and a frequency of the AC voltage component is in a range of 10 kHz to 12 kHz, inclusive.