A building material and a method for coating a substrate for the building material with a coating film having a variety of functions relating to an environment such as mildew resistance, deodorization, antibacterial activity and air purification in addition to the anti-staining effect by having an excellent hydrophilicity. The method for coating the substrate for a building material comprises the steps of; coating a coating material comprising a hydrophilic polymer and a photocatalyst on the substrate, and drying the coating material to form a coating film containing the photocatalyst, wherein the hydrophilic polymer is at least one selected from the group consisting of methyl silicate, liquid glass, colloidal silica, poly(meth)acrylate, and polytetrafluoroethylene graft-polymerized with sulfonic acid, and the photocatalyst is at least one selected from the group consisting of titanium oxide coated with zeolite, titanium oxide coated silica and titanium oxide coated with apatite.