To provide a contamination preventive system for a cathode, in which cost can be reduced as a whole in addition to sure contamination prevention and corrosion prevention.

In the contamination preventive system 10 for the cathode which performs the contamination prevention and corrosion prevention by causing a current to follow between an anode and cathode set in the sea water, a contamination preventive plate PL disposed to cover a wall surface 101 of a contamination prevention object is divided to a cathode plate 11 to be immersed at all times in the sea water and an anode plate 21 to be at least temporarily exposed from the inside of the sea water, and the anode plate 21 serves as the anode by using a conductive plate having higher corrosion resistance to the sea water than the cathode plate 11, for the anode plate 21, and the cathode plate 11 serves as the cathode.