Abstract: A stain preventing method comprises: lining a conductive sheet on an article subjected to stain prevention; arranging an electrode member and a reference electrode in sea water; applying direct electric current using the conductive sheet as an anode and the electrode member as a cathode; allowing a current to flow while controlling a potential difference between the reference electrode and the anode to be in a certain range; and thereby giving electric shock to microorganisms which contact with the conductive sheet so as to prevent adhesion thereof. Stain prevention can be effectively carried out without generating substances harmful for life such as chlorine due to electrolysis of sea water.