To conduct stable, high quality printing by a method in which water is introduced into an electrolytic apparatus having an anode and a cathode to be electrolyzed, and anodic water or cathodic water is supplied into a wetting water supply tank as dampening water.

Filtered water is introduced into the electrolytic cell 5 of an electrolytic apparatus 1 through a conduit 10. In the electrolytic cell 5, an oxidation reaction occurs at an anode 2, while a reduction reaction takes place at a cathode 3. At the anode 2, acid water is obtained in which OH- ion is in shortage, while H+ ion is in abundance. Besides, at the cathode, alkaline water is obtained in which H+ ion is in shortage, while OH- ion is in abundance. In the wetting water supply apparatus 20, the anodic water is supplied into a supply tank 7 through a water supply pipe 13.