PURPOSE: To cover the surface of a plastic with an alga protecting plate at the same time when molding is performed and to make a manufacturing cost lower than that of the case where the alga protecting plate is adhered afterward by placing the alga protecting plate on the inner face of a mold in such a way that the plate fits the inner face of the mold and molding a plastic so as to adhere said alga protecting plate to the plastic.

CONSTITUTION: In manufacturing an FRP boat, a mold 1 is constituted of frames 1a and templates 1b. On the inner face of this mold 1, an alga protecting plate 2 made of cupro-nickel is placed. The alga protecting plate 2 is press-molded so as to fit to the inner face of the mold 1. In addition, if the thickness of the alga protecting plate is large, the template is not necessary under the alga protecting plate, but the alga protecting plate 1 can be directly supported by frames 1a. Then, a reinforcing mat of a glass fiber impregnated with a resin such as an epoxy resin is adhered on the alga protecting plate and an exposed template 1b and an FRP layer 3 with a necessary thickness is formed to adhere the FRP layer 3 on the alga protecting plate 2. By mold releasing after curing, the FRP boat the bottom of which is covered with the alga protecting plate 2 is prepd.