The experiments of cathodic protection have been performed using Daiichi-Tetsuei Maru and four other ships during about one year. The used anode materials were magnesium and zinc plates. Then, following consideration were resulted. (1) In the magnesium anode system it was difficult to control anode current and occured frequently over current, therefoe it was possible to damage the paint of ship and make faster consumption of magnesium anode. (2) But in the zinc anode system current was less than that of magnesium. so it was necessary to have a number of zinc plates but easy to use. And from these performance a basis of cathodic protection and its methods were clear.