An electro-coagulation printer is basically constructed by a rotation drum having a circumferential surface which is electrified in positive potential, a number of electrodes which are selectively electrified in negative potentials, and an ink supply device. The electrodes are aligned to oppositely face with the circumferential surface of the rotation drum with a gap which ranges between thirty and one-hundred microns. The ink supply device is arranged in an incoming side of the rotation drum and supplies liquid ink to be injected onto the circumferential surface of the rotation drum, so that electrification is performed between the electrodes and circumferential surface of the rotation drum with intervention of the liquid ink. Thus, a desired image is formed on the circumferential surface of the rotation drum and is transferred onto a printing material such as a paper. In order to prevent the liquid ink from being stuck to surfaces and peripheral portions of the electrodes, electrolyte containing substantially no coagulating components is supplied to the electrodes. Namely, an ink film thickness regulation roller is provided to regulate the liquid ink to a prescribed thickness (T1) which is smaller than the gap, while a gap of a remaining thickness (T2) is filled with the electrolyte. Thus, electrification is performed in such a manner that the liquid ink having the prescribed thickness is placed on the circumferential surface of the rotation drum, while the electrolyte is placed in contact with the electrodes, wherein the liquid ink and electrolyte are arranged in different phases.