PURPOSE: To effectively utilize the electric corrosion prevention, by perfectly allowing to contact the bottom board of tank with electrolyte and providing with the means lowering the resistivity of base sand at the corrosion preventing method for the outer face of bottom board of tank.

CONSTITUTION: Water in the water feeding vessel 11 is poured in the base sand 3 from the water feeding tube 4 by operating the pump 10 and electrolyte (water or sand) is perfectly allowed to contact with the outer face of bottom board 1 of tank. An electric current for anticorrosion is always turned on the whole outer face of the bottom board 1 from the anode 5. At the time of flowing said water over the drainageway 13, feeding of the water is switched to the upper water feeding tube 12 and outflow of the sand 3 from the drain ditch 13 is prevented. Poured water is returned from the drain ditch 13 to the vessel 11 through the drainpipe 14. Hereby, the bottom board 1 is perfectly allowed to contact with the electrolyte 3 and resistivity of the sand 3 can be lowered and also, effect of electric corrosion prevention is obtained.