To obtain a new compound useful as an antifoulant capable of preventing underwater harmful fouling organisms from adhering without polluting environment.

This compound shown by formula I or II can be produced not only by chemical synthesis but also from extracts of animals, plants and microorganisms containing the compound. As an organism containing the compound, Lemnalia africana may be exemplarily presented. The compounds of formulas I and II can be utilized as an antifoulant by preparing coatings, solutions, emulsions, etc., each using the compound. When used as a coating, the compound is blended with a coating composition so as to prepare an antifouling coating to be applied to the bottoms of ships, underwater structure, water channels for cooling water, etc. The blended amount of the compound is preferably 100 ppm to 5%, more preferably 500 ppm to 1% based on the coating weight.