To increase a developing speed, to eliminate squeezing work and to make high-speed developing compatible with developing that halftone density is uniform.

This device is provided with a supply roller 52 uniformly attaching liquid developer 1 to the surface of a developing roller 51 and an electric field impressing means forming a liquid toner layer 6 in which charge toner grains 5B are aggregated by impressing an electric field on the developer 1 attached to the surface of the roller 51. By making the roller 51 holding the layer 6 in which the grains 5B are aggregated press-contact with a belt-like photoreceptor 7 on which an electrostatic latent image is formed, a developing action is executed within a range that the expression of Vp+V2=V1+Vb is satisfied. Provided that Vp is the surface potential of an electric charge carrier, Vb is the surface potential of a developer supporting body, V1 is the potential of the liquid toner layer from the developer supporting body and V2 is the toner potential of the liquid toner layer from the electric charge carrier. Then, they respectively take a negative and a positive values.