To provide a waterweed-proofing metal plate for those on which waterweed grows, such as a building structure by providing a baking-cured coating film of excellent resistance with waterweed-proofing properties using a certain waterweed-proofing agent.

A thermosetting waterweed-proofing coating composition comprises (A) a thermosetting resin, (B) at least one curing agent selected from an amino resin and a block polyisocyanate compound and (C) S-triazine-based compound, wherein the (C) S-triazine-based compound is present at a level of 0.5-25 pts.wt. based on 100 pts.wt. of a solid content in components (A) and (B) and the (A)/(B) ratio is 100/5-100/60. The (C) S-triazine-based compound is preferably at least one kind selected from 2-methylthio-4,6-bis(ethylamino)-S- triazine, 2-methylthio-4-ethylamino-6-isopropylamino-S-triazine and 2- methylthio-4,6-bis(isopropylamino)-S-triazine.