The authors conducted immersion tests on marine antifouling paint coatings at Tateyama (Chiba prefecture) and Ecchujima (Koto Ward, Tokyo) to investigate efficacy of antifouling coatings. Observation on bio fouling process of barnacles and measurement on leaching rate of antifouling agents from antifouling paint coatings were done throughout the tests. Leaching rate of antifouling agents was measured by apparatus that allows direct measurement from the flat panels. Chlorophyll-a (Chl.a) on immersed flat panels were also measured to estimate bio fouling of algae. And, the authors investigated 2 pure car carriers, container ship and training ship at regular dry dock to clarify actual ship's bio fouling of hull and niche places such as sea chests and rudder, etc. Leaching rate of antifouling agents from tested all marine antifouling paint coatings had been kept over abt.l0pg/cm^2/day for 1 year and barnacles had not stuck on paint coatings using antifouling agents of cuprous oxide(Cu_2O) . On the other hand, barnacles were observed on the coatings using antifouling agents of organic zinc in a minimum of about 100days and an average of about 250 days. The results of the immersion test clarified that efficacy of anti-fouling paint coatings could be estimate on leaching rate of antifouling agents and the rate over abt.10μg/cm^2/day of Cu_2O could prevent barnacles sticking on the anti-fouling paint coatings. Measurement on chlorophyll-a (Chl.a) could estimate quantity of algae stuck on tested panels in beginning of the immersion test. Barnacles and algae were observed on actual ship's hull and niche places of the investigated ships. Anti-fouling paint coatings using anti-fouling agent of Cu_2O were presume to be effective to prevent ship's bio fouling. The results of the investigation of actual ships clarified that ships have various surface conditions of antifouling paint coatings caused by ship's operation and structure that could be considered to cause bio fouling on ships.