Provided is a functional building board having an antimicrobial function, which retains anti-algal and antifungal agents for a long period of time in a coat on an external wall material by using several kinds of antimicrobial composite materials, and is provided with stain-proofing function to wash away stains on the external wall material, and in which the design is not largely restricted by the color of the anti-algal or antifungal paint or by the state of the coat and the design is not influenced by the coating step in the final step. The building board according to the present invention has a coat formed on the surface, and is characterized in that antimicrobial composite materials are included on the surface of the coat and a coat layer containing colloidal silica as a main component is formed, said antimicrobial composite materials being characterized in that they control generation of algae and fungi and said building board being a fiber reinforced cement board.