When the inner surfaces of oil tanks are electrocoated, they show more polarization than without electro coating. But our experiments pointed out those facts that, after cut down of the current the potential of the surface became positive nearly as well as bare steel, and that, the electricity of caoting on the rusty steel increased in several times of clean surface, and that, the influence of oil films sticked on the surfaces to the effects of electrocoating seems very much complicated. It should be worthwhile to investigate to know which is more economic the electro coating or high maintenance currents.