PURPOSE: To enhance workability by forming a fiber-reinforced plastic layer, in which either the periphery is lower then the central part by a certain step or there is no step between the two, on an inclined bed material consisting of a hard, foam plastic board or the like.

CONSTITUTION: In manufacturing an FRP penel joint body 13, a plurality of FRP panels 12, in which a peripheral part 7 of an FRP layer 2 is lower than a central part 8 by a certain step, are fixed on a designated surface in such a manner that the inclination of respective panels follows the same direction. Next, a joint part 9 is filled with resin putty 4, while unhardened resin is applied to the peripheral part 7, laminating it together with reinforcing fibers. By hardening the resin, an FRP layer 3 for joining is formed. After this, an incombustible layer such as concrete and a plasterboard is formed on the FRP layer 2 via the adhesive or the like. With this contrivance, both works for forming a bed with inclination and works for forming the FRP layer on the bed can be executed by a single work-contractor at the same time. In addition, heat insulating performance, water proofing property, damp proofing property, fire resistance, and mechanical strength can be enhanced, and the manufacturing work can be executed, ranging over a wide area and without difficulty.