Cathodic protection has been applied for corrosion prevention method of submerged and mud zone in steel structures at marine environment. However, there are number of unclarified points regarding the protective effectivenss of under adhesion marine growths, to examine adequate protective curret density of contaminated sea water and mud zone. 　Therefor, site tests were conducted in order to clarify unsolved problems regarding aforementioned matters. 　Major finding are as follows; (1)The cathodic protection can stop the corrosion generated during of no protection. (2)Adhesion of sea growths on steel surface were harmless to protective effectiveness of cathodic protection. (3)The protective current density for upper part of submerged zone in contaminated sea water is 1.5～2 time larger than in clean sea water. (4)The required current density of mud zone was about 30 mA/m2 between sea bottom and 10m below sea bottom, and was about 10mA/m2 in the deeper zone than below 10m. (5)Also, application method of adequate protective curret density was proposed which based the pollution index of sea water.