An object of the present invention is to provide organic solvent extracts of red algae laurencia sp., compounds isolated and identified therefrom and an agent for preventing attachment of barnacles comprising them. The present invention relates to a barnacle attachment preventive agent consisting of at least one selected from the group consisting of Laurencin, Thyrsiferol, Magireol A, Omaezallene and Hachijojimallene A and organic solvent extracts of red algae laurencia sp.
