To obtain a composition capable of giving a gypsum wall board imparted with waterproof without loosing strength by including a specific polymer and calcium sulfate at a specific ratio.

This composition is obtained by including (A) 0.5-20 pts.wt., preferably 1-15 pts.wt. of a polymer obtained by the polymerization of (i) 1-100 pts.wt., preferably 20-95 pts.wt. of a (meth)acrylic acid 12-40C alkyl ester [e.g. stearyl (meth)acrylate], (ii) 0-99 pts.wt., preferably 4-79 pts.wt. of an ethylenic unsaturated monomer (e.g. methyl methacrylate) and (iii) 0-15 pts.wt., preferably 1-10 pts.wt. of a monomer (salt) containing an ethylenic unsaturated acid (e.g. methacrylic acid) and (B) 80-99.5 pts.wt. of calcium sulfate. A gypsum wall board obtained by molding the mixture of 0.5-20 pts.wt. of the component A, 80-99.5 pts.wt. of the component B and (C) water is protected by drying.