Disclosed is a developing unit of a wet electrophotographic printer, which has a photoconductive drum, a developing roller for transferring a developer onto the photoconductive drum, a cartridge for storing a developer therein, a developer guide disposed at a predetermined distance away from an inner wall of the cartridge, and having a developer supplying path defined therein to guide the developer in the cartridge to the developing roller, and a developer pumping means disposed in the cartridge to pump up the developer in the cartridge toward the developing roller.