To make stable developing processing possible from a highlight image reduced in the adhesion area of toner particles to a shadow image wide in the adhesion area of toner particles.

A time becoming a peak in the adhesion amount of toner particles to the highlight image reduced in the adhesion area of toner particles is set to a developing time, the length X of a main electrode plate 64 is set to 100 mm or less on the basis of the developing time and the feed speed (v) of a printing plate 12, and the interval between the leading end on the downstream side of the main electrode plate and a squeeze roller 56 is set to 50 mm or more. By this constitution, the adhesion amount of toner particles to the printing plate 12 passing the position opposed to the main electrode plate becomes the peak corresponding to an image area or becomes an amount near to the peak on this side of the peak. Further, since the protruberance of a liquid developer does not reach the main electrode plate on this side of the squeeze roller, stable developing treatment becomes possible and the lowering of image quality caused by the back flow of the liquid developer can be prevented.